docs: support markdown (MR 2361)

Let's write all new documentation in markdown, and convert existing ones
at some point. I think we are all much more familiar with markdown than
with the rst format.

.ci/docs.sh

@@ -4,10 +4,11 @@
 # https://postmarketos.org/pmb-ci
 
 
-# Ensure sphinx_rtd_theme is installed
+# Install sphinx + extensions when running in CI
 if [ "$(id -u)" = 0 ]; then
 	set -x
 	apk -q add \
+		py3-myst-parser \
 		py3-sphinx_rtd_theme \
 		py3-sphinxcontrib-autoprogram
 	exec su "${TESTUSER:-build}" -c "sh -e $0"

docs/conf.py

@@ -34,6 +34,7 @@ version = ".".join(release.split(".")[:3])
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#general-configuration
 
 extensions = [
+    "myst_parser",
     "sphinx.ext.autodoc",
     "sphinx.ext.autosummary",
     "sphinx.ext.doctest",
@@ -42,6 +43,7 @@ extensions = [
 
 templates_path = ["_templates"]
 exclude_patterns = ["_build", "Thumbs.db", ".DS_Store"]
+source_suffix = [".rst", ".md"]
 
 # -- Options for HTML output -------------------------------------------------
 html_theme = "sphinx_rtd_theme"