Make MkDocs TOC two-level

MkDocs states that, for the readthedocs theme, only two levels of navigation are supported - see https://www.mkdocs.org/user-guide/choosing-your-theme/#readthedocs.

This commit changes the TOC to using only two levels of navigation.

Author: mpilgrem

mkdocs.yml

@@ -40,15 +40,16 @@ pages:
 - Advanced documentation:
   - Build overview: build_overview.md
 - Project documentation:
+- For Contributors:
   - Contributors guide: CONTRIBUTING.md
-  - Maintainer guide:
-    - Releases: maintainers/releases.md
-    - Maintainer team process: maintainers/team_process.md
-    - Add GHC version: maintainers/ghc.md
-    - Docker images: maintainers/docker.md
-    - Upgrading MSYS2: maintainers/msys.md
-    - HaskellStack.org: maintainers/haskellstack.org.md
-  - Signing key: SIGNING_KEY.md
+- For Maintainers:
+  - Releases: maintainers/releases.md
+  - Maintainer team process: maintainers/team_process.md
+  - Add GHC version: maintainers/ghc.md
+  - Docker images: maintainers/docker.md
+  - Upgrading MSYS2: maintainers/msys.md
+  - HaskellStack.org: maintainers/haskellstack.org.md
+- Signing key: SIGNING_KEY.md
 - Glossary: glossary.md
 
 markdown_extensions: