fix(ci/docs): point action at docs requirements

mhausenblas/mkdocs-deploy-gh-pages needs a requirements file, else it
tries to install the repo-level requirements, which typically are not
needed for the docs site, see:
mhausenblas/mkdocs-deploy-gh-pages#53

introduce requirements file specific to the docs site, fixing an issue
with trying to install the dev-requirements.txt

the underlying issue was with trying to install black into the Docker
image used by this GitHub Action (squidfunk/mkdocs-material:7.1.8), see:
https://github.com/mhausenblas/mkdocs-deploy-gh-pages/blob/master/Dockerfile

recent versions of black depend on regex, which has a history of trouble
with Linux wheel availability and support

* psf/black#1207
* https://bitbucket.org/mrabarnett/mrab-regex/issues/343/wheel-for-linux
* https://bitbucket.org/mrabarnett/mrab-regex/issues/349/no-module-named-regex_regex-regex-is-not-a

the Docker image does not have gcc available; rather than installing a
build chain to build regex to enable the install of black, which isn't
even used by the docs site, the simpler fix was to create a
docs/requirements.txt and point the GitHub Action at it instead

similar report on the Action repo:
mhausenblas/mkdocs-deploy-gh-pages#105

.github/workflows/mkdocs.yml

@@ -16,5 +16,5 @@ jobs:
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@master
         env:
-          REQUIREMENTS: dev-requirements.txt
+          REQUIREMENTS: docs/requirements.txt
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

docs/requirements.txt

@@ -0,0 +1,2 @@
+mkdocs
+mkdocs-material