fix: add missing doc entry for content management plugin

[ruanyl]

Description

Add missing doc entry to docs/_sidebar.md

Issues Resolved

Screenshot

Testing the changes

Changelog

• skip

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

[opensearch-trigger-bot]

The backport to 2.x failed:

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

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch-Dashboards/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch-Dashboards/backport-2.x
# Create a new branch
git switch --create backport/backport-7695-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 65ecd3ba9982f65cde6250085eb119931efb360b
# Push it to GitHub
git push --set-upstream origin backport/backport-7695-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch-Dashboards/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-7695-to-2.x.

[ruanyl]

@AMoo-Miki backport 2.x failed, seems 2.x branch doesn't have docs/_sidebar.md, perhaps shouldn't backport to 2.x?

[AMoo-Miki]

Thanks @ruanyl. Will wait to hear from @ashwin-pc on why #5977 wasn't backported.