Update CI scripts and config to use Nox and remove old theme cruft

.github/workflows/build.yaml

@@ -51,7 +51,7 @@ jobs:
       run: ./ci/build.sh
     - name: Postprocess site
       shell: bash
-      run: make multidocs
+      run: nox -s deploy
     - name: Deploy to GitHub Pages
       if: env.IS_DEPLOY == '1'
       uses: JamesIves/github-pages-deploy-action@v4

.github/workflows/check.yaml

@@ -42,4 +42,4 @@ jobs:
       shell: bash
       run: pip list
     - name: Run Linkcheck
-      run: make linkcheck
+      run: nox -s linkcheck

ci/build.sh

@@ -1,3 +1,3 @@
 #!/bin/bash -ex
 
-make docs
+nox -s build

ci/install.sh

@@ -1,20 +1,4 @@
 #!/bin/bash -ex
 
 python3 -m pip install --upgrade pip setuptools wheel
-python3 -m pip install --upgrade -r requirements.txt
-
-# You can uncomment the command below to use a different version of the theme (but return it to the correct one before merging)
-# python3 -m pip install --upgrade --force-reinstall --no-deps git+https://github.com/spyder-ide/spyder-docs-sphinx-theme.git@develop_spyder
-
-# Configure the username and email so Git doesn't complain
-if [ "${CI-}" = "true" ]; then
-    git config --global user.email "[email protected]"
-    git config --global user.name "Spyder-Docs Deploy Bot"
-fi
-
-# Needed to build PR previews using Netlify with the different versions of the docs available
-if [ "${NETLIFY-}" = "true" ]; then
-    git config remote.upstream.url >&- || git remote add upstream https://github.com/spyder-ide/spyder-docs.git
-    git fetch --all
-    git update-ref -m "reset: Update master to latest commit" refs/heads/master upstream/master
-fi
+python3 -m pip install --upgrade nox