Added documentation for select entries, map-to-list, and trucate processors.

[kkondaka]

Description

Added documentation for

• select-entries processor
• truncate processor
• map-to-list processor
  Updated documentation for
• file sink
• s3 sink
• kafka buffer

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

Checklist

• [X ] By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin.
  For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[Naarcha-AWS]

Going to remove this in favor of #6680.

[Naarcha-AWS]

Can confirm the map-to-list will work once this is merged in.

[dlvenable]

I think we can be more explicit here.

The select_entries processor selects entries from a Data Prepper event. Only the selected entries will remain in the event and all other ones will be removed.

[dlvenable]

This is not required anymore.

By default, it will truncate all fields.

source_keys | Not | String list | The list of sources keys that will be modified by the processor. The default value is an empty list which indicates truncating all values.

[Naarcha-AWS]

@natebower: This is ready for your review.

[natebower]

@kkondaka @Naarcha-AWS Please see my comments and changes and let me know if you have any questions. I'd like to see reread lines 25, 62, and 69 in truncate.md before approving. Thanks!

[natebower]

@Naarcha-AWS Just one comma to add. Otherwise, LGTM!

[opensearch-trigger-bot]

The backport to 2.12 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.12 2.12
# Navigate to the new working tree
pushd ../.worktrees/backport-2.12
# Create a new branch
git switch --create backport/backport-6660-to-2.12
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e697522b99a1b77b83362d328e0ae940326e445e
# Push it to GitHub
git push --set-upstream origin backport/backport-6660-to-2.12
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.12

Then, create a pull request where the base branch is 2.12 and the compare/head branch is backport/backport-6660-to-2.12.