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>

Documentation/technical/hash-function-transition.txt

@@ -148,8 +148,8 @@ Detailed Design
 Repository format extension
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 A SHA-256 repository uses repository format version `1` (see
-Documentation/technical/repository-version.txt) with extensions
-`objectFormat` and `compatObjectFormat`:
+linkgit:gitrepository-layout[5]) with `extensions.objectFormat` and
+`extensions.compatObjectFormat` (see linkgit:git-config[1]) set to:
 
 	[core]
 		repositoryFormatVersion = 1

Documentation/technical/partial-clone.txt

@@ -102,7 +102,7 @@ or commits that reference missing trees.
 - On the client a repository extension is added to the local config to
   prevent older versions of git from failing mid-operation because of
   missing objects that they cannot handle.
-  See "extensions.partialClone" in Documentation/technical/repository-version.txt"
+  See `extensions.partialClone` in linkgit:git-config[1].
 
 
 Handling Missing Objects

Documentation/technical/repository-version.txt

@@ -65,44 +65,6 @@ Note that if no extensions are specified in the config file, then
 provides no benefit, and makes the repository incompatible with older
 implementations of git).
 
-This document will serve as the master list for extensions. Any
-implementation wishing to define a new extension should make a note of
-it here, in order to claim the name.
-
-The defined extensions are:
-
-==== `noop`
-
-This extension does not change git's behavior at all. It is useful only
-for testing format-1 compatibility.
-
-==== `preciousObjects`
-
-When the config key `extensions.preciousObjects` is set to `true`,
-objects in the repository MUST NOT be deleted (e.g., by `git-prune` or
-`git repack -d`).
-
-==== `partialClone`
-
-When the config key `extensions.partialClone` is set, it indicates
-that the repo was created with a partial clone (or later performed
-a partial fetch) and that the remote may have omitted sending
-certain unwanted objects.  Such a remote is called a "promisor remote"
-and it promises that all such omitted objects can be fetched from it
-in the future.
-
-The value of this key is the name of the promisor remote.
-
-==== `worktreeConfig`
-
-If set, by default "git config" reads from both "config" and
-"config.worktree" files from GIT_DIR in that order. In
-multiple working directory mode, "config" file is shared while
-"config.worktree" is per-working directory (i.e., it's in
-GIT_COMMON_DIR/worktrees/<id>/config.worktree)
-
-==== `refStorage`
-
-Specifies the file format for the ref database. The valid values are
-`files` (loose references with a packed-refs file) and `reftable` (see
-Documentation/technical/reftable.txt).
+The defined extensions are given in the `extensions.*` section of
+linkgit:git-config[1]. Any implementation wishing to define a new
+extension should make a note of it there, in order to claim the name.