doc: consolidate extensions in git-config documentation

doc: consolidate extensions in git-config documentation

The `technical/repository-version.txt` document originally served as the
master list for extensions, requiring that any new extensions be defined
there. However, the `config/extensions.txt` file was introduced later
and has since become the de facto location for describing extensions,
with several extensions listed there but missing from
`repository-version.txt`.

This consolidates all extension definitions into `config/extensions.txt`,
making it the authoritative source for extensions. The references in
`repository-version.txt` are updated to point to `config/extensions.txt`,
and cross-references to related documentation such as
`gitrepository-layout[5]` and `git-config[1]` are added.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Caleb White <cdwhite3@pm.me>
Signed-off-by: Taylor Blau <me@ttaylorr.com>