[ILM][DOCS] Add Documentation for new ILM security privileges

[talevy]

This is a tracking issue for adding documentation discussing the new manage_ilm, read_ilm, and
changes to view_index_metadata changes in privileges made here: #36493

What needs to be done:

• Update Authorization mentions in the ILM API docs mentioning the need for the manage privilege. This needs to be changed to be custom per api-endpoint where different cluster and/or index privileges are needed
• Update stack-overview documentation on security privileges to mention changes in more detail. https://github.com/elastic/stack-docs

[elasticmachine]

Pinging @elastic/es-core-features

[talevy]

Closed by elastic/stack-docs#172 and #36749