Use fallback distro image if not available

[christophebedard]

Kilted was added in #5401 and a kilted branch was created from that. Multiversion doc builds (make multiversion) currently fail because there is no source/Releases/kilted-small.png file: https://build.ros.org/job/doc_ros2doc/1694/console. This wasn't caught here because we only test building docs for the current branch (make html) and images are not used in that case: it's set in the conf.py when doing a multiversion build

ros2_documentation/conf.py

Line 305 in 6f7f313

app.config.html_logo = 'source/Releases/' + app.config.smv_current_version + '-small.png'

.

In this case, I simply copied the Rolling image to unblock multiversion docs builds: #5418.

Consider using a fallback image if the $distro-small.png image is not available. It may be worth triggering a warning in that case so that it's visible enough. However, if the warning is only shown on the build.ros.org job, it may not be visible enough. We could just make a GitHub CI job here fail if the image is not available.