ci(docs): point the pip cache at the docs requirements file

The default cache glob matches requirements.txt or pyproject.toml, and the
documentation toolchain is deliberately kept separate from the application's —
so setup-python found nothing and failed the job before it could build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/docs.yml

@@ -42,6 +42,10 @@ jobs:
         with:
           python-version: '3.12'
           cache: pip
+          # The default cache glob only looks for requirements.txt or
+          # pyproject.toml, neither of which exists here — the docs toolchain is
+          # kept separate from the application's.
+          cache-dependency-path: requirements-docs.txt
       - run: pip install -r requirements-docs.txt
       # --strict turns warnings into errors, which is the point of running this
       # on a pull request at all: a link to a page that no longer exists should
@@ -60,6 +64,10 @@ jobs:
         with:
           python-version: '3.12'
           cache: pip
+          # The default cache glob only looks for requirements.txt or
+          # pyproject.toml, neither of which exists here — the docs toolchain is
+          # kept separate from the application's.
+          cache-dependency-path: requirements-docs.txt
       - run: pip install -r requirements-docs.txt
 
       - name: Derive version from the shipped package