Update integration docs for individual AWS/Amazon services

[colleenmcginnis]

Note: In this issue I use "AWS integrations" to refer to integrations that fall under the "AWS" menu item even though the name of some of them start with "Amazon" instead of "AWS".

Background

In #3308 we updated docs for two AWS integrations to align with the new documentation guidelines and establish the relationship between the AWS integration/package ("AWS") and integrations for individual AWS services (for example, "AWS CloudFront").

Now we should update the docs for all AWS integrations for individual services to follow the same format as the updated "AWS CloudFront" integration docs.

Guidelines for documenting individual AWS services

Follow the general guidelines in documentation_guidelines.md (subject to change, but should be near final).

For the "Overview" section, you may need to do a little research on what each AWS service does to explain the value of the integration and come up with an example. In my [limited] experience, I found the screenshots of dashboards to be a good source of inspiration for coming up with an example. Then the reviewer can suggest changes to the example if necessary.

In addition to the general guidelines, each AWS integration should include this specific language:

• In the "Requirements" section, include references back to the "AWS" page for more details:

   Before using any AWS integration you will need:
  
   * **AWS Credentials** to connect with your AWS account.
   * **AWS Permissions** to make sure the user you're using to connect has permission to share the relevant data.
  
   For more details about these requirements, see the **AWS** integration documentation.

• In the "Setup" section, advise the reader when to use this individual integration vs. the AWS integration:

   Use this integration if you only need to collect data from the <service name> service.
  
   If you want to collect data from two or more AWS services, consider using the **AWS** integration.
   When you configure the AWS integration, you can collect data from as many AWS services as you'd like.

Services to document

• Billing Update Billing.md #3584
• CloudFront [docs] Add context to AWS integration documentation #3308
• CloudTrail Update CloudTrail - First draft of AWS integration edits #3611
• CloudWatch First draft of CloudWatch edits #3614
• DynamoDB (Update DynamoDB- first draft of AWS integration edits #3633
• EBS (Update EBS- first draft of AWS integration edits #3634)
• EC2 (Update EC2 - first draft of AWS integration edits #3638)
• ELB (Update ELB - first draft of AWS integration edits #3656)
• Fargate (Update AWS Fargate - first draft of AWS integration edits #3730)
• Network Firewall (Update Network Firewall - added new content based on the new doc guidelines #3732)
• Lambda (Update Lambda - first draft of AWS integration edits #3749)
• NAT Gateway (Update NAT Gateway - first draft of AWS integration edits #3751)
• RDS (Update RDS - First draft of AWS integration edits #3752)
• Route 53 (Update Route 53 - First draft of AWS integration edits #3756)
• S3 (Update S3 - First draft of AWS integration edits #3804)
• S3 Storage Lens ( Update S3 Storage Lens- First draft of AWS integration edits #3806)
• SNS (Update SNS - First draft of AWS integration edits #3808)
• SQS (Update SQS First draft of AWS integration edits #3809)
• Transit Gateway ( Update AWS Transit Gateway - First draft of AWS integration edits #3810)
• Usage (Update AWS Usage - First draft of AWS integration edits #3824)
• VPC Flow (Update vpcflow.md - First draft of AWS integration edits #3826)
• VPN (Update VPN.MD First draft of AWS integration edits #3828)
• WAF (Update WAF - First draft of AWS integration edits #3833)
• Custom (Update Readme - First draft of AWS integration edits #3834)

PR template for each service

If it's helpful, you can copy and paste this template into each PR to track progress:

## What does this PR do?

From https://github.com/elastic/integrations/issues/3572:

>In https://github.com/elastic/integrations/pull/3308 we updated docs for two AWS integrations to align with the new documentation guidelines and establish the relationship between the AWS integration/package ("AWS") and integrations for individual AWS services (for example, "AWS CloudFront").
>
>Now we should update the docs for all AWS integrations for individual services to follow the same format as the updated "AWS CloudFront" integration docs.

This PR adds more context the AWS <service> integration including:

- [ ] Adds context to the "Overview" including a link to the relevant AWS page and an example
- [ ] Lists the types of "Data streams" for the service
- [ ] "Requirements" points back to "AWS" for detailed information on credentials and permissions
- [ ] "Requirements" includes any other service-specific requirements
- [ ] "Setup" establishes a relationship between the AWS integration/package ("AWS") and this integration
- [ ] Includes "Reference" sections

## For the reviewer

<!-- anything to highlight for the reviewer -->

## Checklist

- [ ] I have reviewed [tips for building integrations](https://github.com/elastic/integrations/blob/main/docs/tips_for_building_integrations.md) and this pull request is aligned with them.
- [ ] I have verified that all data streams collect metrics or logs.
- [ ] I have added an entry to my package's `changelog.yml` file.
- [ ] I have verified that Kibana version constraints are current according to [guidelines](https://github.com/elastic/elastic-package/blob/master/docs/howto/stack_version_support.md#when-to-update-the-condition).

## Author's Checklist

- [ ] Review by docs team 
- [ ] Review by integrations team

## Related issues

- https://github.com/elastic/integrations/issues/3572

[colleenmcginnis]

Note: The updated documentation_guidelines.md were merged in #3433 🎉 so the new link is: https://github.com/elastic/integrations/blob/main/docs/documentation_guidelines.md. I also updated this link in the original post above.