ci: Run docs job in chroot

See commits for details.

Fixes #2352 (closed)

• TODO: Add new option to https://wiki.postmarketos.org/wiki/Pmbootstrap_CI - will add later once things got approved

.ci/docs.sh

 #!/bin/sh -e
 # Description: create documentation with sphinx
-# Options: native
+# Artifacts: public/
 # https://postmarketos.org/pmb-ci