Spring Move reference docs and build configuration to dedicated module

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 #28451)