ci: Run docs job in chroot

Use the new "Artifacts" directive for .ci/docs.sh and run the job in the
pmbootstrap chroot to avoid needing to install sphinx dependencies on
the host.

Fixes #2352

.ci/docs.sh

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

.gitlab-ci.yml

@@ -76,7 +76,6 @@ mypy:
 docs:
   stage: lint
   script:
-    - apk add py3-sphinx py3-sphinx_rtd_theme
     - ".ci/docs.sh"
   artifacts:
     paths: