Document each store type on its own page

This makes for more useful manual table of contents, that displays the
information at a glance.

The `nix help-stores` command is kept as-is, even though it will show up
in the manual with the same information as these pages due to the way it
is written as a "`--help`-style" command. Deciding what to do with that
command is left for a later PR.

This change also lists all store types at the top of the respective overview page.

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems

src/libexpr/primops.cc

@@ -4460,7 +4460,7 @@ void EvalState::createBaseEnv()
         .doc = R"(
           Logical file system location of the [Nix store](@docroot@/glossary.md#gloss-store) currently in use.
 
-          This value is determined by the `store` parameter in [Store URLs](@docroot@/command-ref/new-cli/nix3-help-stores.md):
+          This value is determined by the `store` parameter in [Store URLs](@docroot@/store/types/index.md#store-url-format):
 
           ```shell-session
           $ nix-instantiate --store 'dummy://?store=/blah' --eval --expr builtins.storeDir