Mirrors config: support multiple repos and no repos, make it more consistent and document it

See individual commits for details.

This is needed for bpo to use pmbootstrap v3.

Close #2397 (closed)

Fix #2405 (closed)

.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"