[Filebeat] aws-s3 - Document _id generation behavior

[andrewkroh]

Preview link: https://beats_bk_42127.docs-preview.app.elstc.co/guide/en/beats/filebeat/master/filebeat-input-aws-s3.html

Proposed commit message

Document the details about how the Filebeat aws-s3 input generates
Elasticsearch document _id values.

Add a subsection for the configuration examples.

Move "Common configuration" section immediately after the input
configuration options.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Relates [x-pack][filebeat] incorporate LastModified into s3 event _id generation #42078 (my updated text includes Last-Modified time in the behavioral description)

Screenshots

Before	After

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @andrewkroh? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[kilfoyle]

LGTM! 🎸