Jobs based on custom runners: documentation and configuration placeholder

Jobs based on custom runners: documentation and configuration placeholder

As described in the included documentation, the "custom runner" jobs
extend the GitLab CI jobs already in place.  One of their primary
goals of catching and preventing regressions on a wider number of host
systems than the ones provided by GitLab's shared runners.

This sets the stage in which other community members can add their own
machine configuration documentation/scripts, and accompanying job
definitions.  As a general rule, those newly added contributed jobs
should run as "non-gating", until their reliability is verified (AKA
"allow_failure: true").

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210630012619.115262-2-crosa@redhat.com>
Message-Id: <20210709143005.1554-2-alex.bennee@linaro.org>