7.5 Beats documentation roadmap

[dedemorton]

Changed or deprecated features:

• Deprecate beats central management docs: Deprecate Beats central management docs #14045 [DeDe] - [skip ci][DOCS] Mark Beats central management as discontinued kibana#49423 and [DOCS] Deprecate central management #14104 (in progress)

Release-related docs:

• Document breaking changes: [DOCS] Add placeholder files for 7.5 BCs and highlights #14398 (in progress)
• Document release highlights. [DOCS] Add placeholder files for 7.5 BCs and highlights #14398 (in progress)
• Review and, if necessary, edit the release notes (changelog). [DeDe]

Other changes:

• Refactor processor docs by moving processors under the code modules. [DOCS] Create separate topics for processor docs #14410
• Refactor input docs by moving inputs under code modules. [DOCS] Refactor output docs #14724