Use Plone Sphinx Theme (#1770)

* Use Plone Sphinx Theme for the theme
- Update requirements.txt, removing pins and obsolete packages
- Move vale styles into proper location

* Sync conf.py with Plone Sphinx Theme options

* Fix configuration for version and switcher
Remove conflicting templates

* Remove conflicting static assets and template files, and adjust configuration accordingly.

* Remove obsolete Netlify files

* Align version switcher to match this version

* Add YouTube social icon.
Remove obsolete comments.

* Revert `make distclean` to merely purge the venv

* Fix scrollspy

* Whoops, just delete this monkeypatch, as it comes straight from PST.

* Move icon-links from primary sidebar to footer_content_items.
- Remove `version` from the whole-page footer.
- Sort html_theme_options

Makefile

@@ -30,13 +30,8 @@ clean:  ## Clean docs build directory
 	cd $(DOCS_DIR) && rm -rf $(BUILDDIR)/
 
 .PHONY: distclean
-distclean:  ## Clean docs build directory and Python virtual environment, then install requirements
-	cd $(DOCS_DIR) && rm -rf $(BUILDDIR)/
-	python3 -m venv venv --clear
-	venv/bin/pip install -r requirements-initial.txt
-	venv/bin/pip install -r requirements.txt
-	@echo
-	@echo "Installation of requirements completed."
+distclean: clean ## Clean docs build directory and Python virtual environment
+	rm -rf venv
 
 venv/bin/python:  ## Setup up Python virtual environment and install requirements
 	python3 -m venv venv