Overview

Split the monolithic src/mongo/db/repl/FCV_AND_FEATURE_FLAG_README.md (~1370 lines) into separate, cross-linked documents to make the content more digestible and navigable. No content/wording changes — purely a structural split plus reference fixups.

Background

The doc lived under src/mongo/db/repl/, but the code it documents (feature_flag.h/.cpp, feature_compatibility_version_*) actually lives directly under src/mongo/db/, and the setFCV command lives under src/mongo/db/commands/. The new docs follow the existing README_.md naming convention already used in src/mongo/db/ (e.g. README_version_context.md, README_default_max_time_ms.md).

Scope of Work

* Split into 7 topic files in src/mongo/db/, one per top-level section of the original doc:
** README_fcv_overview.md
** README_setfcv_command.md
** README_fcv_generic_references.md
** README_fcv_constants_generation.md
** README_feature_flags.md
** README_multiversion_testing.md
** README_fcv_feature_flag_rules.md
* Add a "FCV and Feature Flags" doc-index section to src/mongo/db/README.md linking to all of the above, plus entries for the two pre-existing but previously unindexed README_version_context.md and README_default_max_time_ms.md.
* Delete src/mongo/db/repl/FCV_AND_FEATURE_FLAG_README.md and update all known references:
** src/mongo/db/repl/README.md
** src/mongo/db/repl/OWNERS.yml (move entry to src/mongo/db/OWNERS.yml)
** src/mongo/db/query/README_query_feature_flags.md
** docs/evergreen-testing/multiversion.md
** buildscripts/idl/idl_schema.yml

Acceptance Criteria

* No content/wording changes beyond path and link fixups.
* All internal section anchors and cross-file links resolve correctly.
* bazel run format passes clean.

Related

* SERVER-125642 (Documentation Updates)