manual: Contributing -> Development, Hacking -> Building (#9014)

* manual: Contributing -> Development, Hacking -> Building

what's currently called "hacking" are really instructions for setting up
a development environment and compiling from source. we have
a contribution guide in the repo (which rightly focuses on GitHub
workflows), and the material in the manual is more about working
on the code itself.

since we'd otherwise have three headings that amount to "Building Nix",
this change also moves the "classic Nix" instructions to the top.

we may want to reorganise this in the future, and bring
contributor-oriented information closer to the code, but for now let's
stick to more accurate names to ease navigation.

src/libexpr/primops/fetchTree.cc

@@ -383,7 +383,7 @@ static RegisterPrimOp primop_fetchTree({
       - `"mercurial"`
 
      *input* can also be a [URL-like reference](@docroot@/command-ref/new-cli/nix3-flake.md#flake-references).
-     The additional input types and the URL-like syntax requires the [`flakes` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-flakes) to be enabled.
+     The additional input types and the URL-like syntax requires the [`flakes` experimental feature](@docroot@/development/experimental-features.md#xp-feature-flakes) to be enabled.
 
       > **Example**
       >
@@ -670,12 +670,12 @@ static RegisterPrimOp primop_fetchGit({
 
         Whether to check `rev` for a signature matching `publicKey` or `publicKeys`.
         If `verifyCommit` is enabled, then `fetchGit` cannot use a local repository with uncommitted changes.
-        Requires the [`verified-fetches` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-verified-fetches).
+        Requires the [`verified-fetches` experimental feature](@docroot@/development/experimental-features.md#xp-feature-verified-fetches).
 
       - `publicKey`
 
         The public key against which `rev` is verified if `verifyCommit` is enabled.
-        Requires the [`verified-fetches` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-verified-fetches).
+        Requires the [`verified-fetches` experimental feature](@docroot@/development/experimental-features.md#xp-feature-verified-fetches).
 
       - `keytype` (default: `"ssh-ed25519"`)
 
@@ -687,7 +687,7 @@ static RegisterPrimOp primop_fetchGit({
         - `"ssh-ed25519"`
         - `"ssh-ed25519-sk"`
         - `"ssh-rsa"`
-        Requires the [`verified-fetches` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-verified-fetches).
+        Requires the [`verified-fetches` experimental feature](@docroot@/development/experimental-features.md#xp-feature-verified-fetches).
 
       - `publicKeys`
 
@@ -701,7 +701,7 @@ static RegisterPrimOp primop_fetchGit({
         }
         ```
 
-        Requires the [`verified-fetches` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-verified-fetches).
+        Requires the [`verified-fetches` experimental feature](@docroot@/development/experimental-features.md#xp-feature-verified-fetches).
 
 
       Here are some examples of how to use `fetchGit`.