Merge bitcoin#24509: doc: Add guix prefix for changes to reproducible builds

f1f994a doc: Add `guix` prefix for changes to reproducible builds (Hennadii Stepanov)

Pull request description:

  Most of contributors already use the `guix:` prefix for changes to `contrib/guix`. Also `guix` is shorter than `build`, and it is more focused/specific.

ACKs for top commit:
  fanquake:
    ACK f1f994a

Tree-SHA512: 3f754e80802ec4e871b099ce1f0877e34ecc4816fbe9c49bfd2a7368ef79fed9edf6c65f38eedef2a87367fdc911dc548e0def422d80b66a91ce2e5f35826032

Author: fanquake

CONTRIBUTING.md

@@ -153,7 +153,8 @@ the pull request affects. Valid areas as:
   - `test`, `qa` or `ci` for changes to the unit tests, QA tests or CI code
   - `util` or `lib` for changes to the utils or libraries
   - `wallet` for changes to the wallet code
-  - `build` for changes to the GNU Autotools or reproducible builds
+  - `build` for changes to the GNU Autotools or MSVC builds
+  - `guix` for changes to the GUIX reproducible builds
 
 Examples: