Elasticsearch reference cleanup

[lcawl]

The reference information that was added to the elasticsearch repo in elastic/elasticsearch#123507 requires the following clean-up;

• Add content to blank Plugin management page
• Consolidate the ECE, ESS, and Serverless setting info into the stateful Elasticsearch configuration settings. [DOCS] Restructure user settings reference pages elasticsearch#125000
• Fix external URI images commented out in elastic/elasticsearch@341ba7c
• Add links from reference landing pages to overview/conceptual counterparts in docs-content.
  • Partially addressed by elasticsearch/pull/123733

[charlotte-hoblik]

Hi @lcawl, I started working on this issue today. I’ve created a pull request for the images sub-issue and have begun looking into the “Add content to blank Plugin management page” task.

[charlotte-hoblik]

I’ve created two pull requests related to the Plugin Management documentation changes:
• PR #124578 in the elasticsearch repo for the documentation updates.
• PR #734 in the docs-content repo to update links pointing to the old plugin management pages.

@leemthompo @lcawl, since you have the context for this task, could you review them when you have time? Thanks! :)