CI: Move the documentation check to a separate job

[lslezak]

CI Improvements

• Run the yardoc check and the D-Bus documentation check only when a respective file is updated
• This saves some unnecessary builds and makes CI checks faster
• Cache the installed RubyGems, this reduces the installation time from ~1m40s to ~4s (!)
• Use the smaller Tumbleweed base image - it needs to install more packages (8s -> 24s = +16s), but the Docker image download is much faster (~2m -> 10s = -110s) so overall it is about 1-2 minutes faster

[coveralls]

coverage: 72.134%. remained the same when pulling ef48130 on ci-yardoc into e9bff87 on master.