Point to requirements file when building docs

This applies both to the RTD builds and to the GHA builds.

Signed-off-by: Rodrigo Tobar <[email protected]>

.github/workflows/build-docs.yml

@@ -12,4 +12,4 @@ jobs:
     - uses: ammaraskar/sphinx-action@master
       with:
         docs-folder: "doc"
-        pre-build-command: 'pip install -U sphinx_rtd_theme docutils'
+        pre-build-command: 'pip install -r doc/requirements.txt'

.readthedocs.yaml

@@ -10,3 +10,7 @@ submodules:
 sphinx:
   configuration: doc/conf.py
   fail_on_warning: true
+
+python:
+  install:
+    - requirements: doc/requirements.txt