[DOC] Restructure aggregations pages into subpages

[vagimeli]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.
Improve readability by breaking up dense aggregations documentation into logical subpages.

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
Backport to latest

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
#7115 (See dev's comment about subpages)