ref(tests): test docs/ before building any images to fail faster

An error in the reStructuredText sources under docs/ is a hard error
that will stop the tests, but the test scripts will needlessly build
all the docker images before finding that out. This change tests the
docs first, then moves on to component tests.
Merge pull request #2266 from mboersma/test-docs-first

ref(tests): test docs/ before building any images to fail faster