doc: Clean up and modernize Sphinx configuration (fixes #463) (#465)

* Switch from setuptools' pkg_resources to importlib.metadata.

The remaining changes are based on a fresh invocation of sphinx-quickstart (version 7.2.6), trying to format our existing modifications in the same style.

* Update string literals to assume UTF-8 default.
* Remove commented Sphinx configuration options.
* Remove Sphinx options which are at their default.
* Reorder, project settings first.
* Remove explanatory comments on configuration options.
* Include section headers as generated via sphinx-quickstart.
* Remove default HTML theme option.
* Reorder (sort) extensions.
* Fix undefined default language.
* Rename modernized Sphinx options.
* Clean up section headings and move autodoc option down.
* Import Makefile generated by sphinx-quickstart.

Author: acolomb

doc/Makefile

@@ -0,0 +1,20 @@
+# Minimal makefile for Sphinx documentation
+#
+
+# You can set these variables from the command line, and also
+# from the environment for the first two.
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+SOURCEDIR     = .
+BUILDDIR      = _build
+
+# Put it first so that "make" without argument is like "make help".
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: help Makefile
+
+# Catch-all target: route all unknown targets to Sphinx using the new
+# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+%: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)