Move reference docs and build configuration to dedicated module

[bclozel]

Currently, the reference documentation is hosted in the root module, as well as all the build configuration related to doc artifacts.
We should move this to a dedicated module, "framework-docs", to bring the following benefits:

• clear separation of build configuration between the docs and the rest of the build
• allow "framework-docs" to depend on "spring-*" modules so that actual Java sources can be included as snippets in the reference docs
• using the convention based approach will require anchor changes, which are required anyway for deduplication (which currently blocks Reference PDF containing full docs not available #28451)