Add a check for videos format in CI

[djhi]

Problem

We had many issues in the past with incompatible video formats or codecs on video assets used in the documentation (e.g. not readable on Firefox or Ubuntu+Chrome)

We risk doing the same error in the future, as our screen capture tools seem to produce erroneous videos by default

Solution

Add a CI step to check the video asset files

How To Test

• Create a video with a wrong format (use something like https://convertio.co/ to convert a video into HEVC format)
• run the script: make check-documentation-videos-format
• It should print a warning:

Invalid codec for ./docs/img/test/test.mp4
Convert it to avc1 with:
ffmpeg -i ./docs/img/test/test.mp4 -c:v libx264 -c:a copy ./docs/img/test/test_avc1.mp4
rm ./docs/img/test/test.mp4
mv ./docs/img/test/test_avc1.mp4 ./docs/img/test/test.mp4
make: *** [Makefile:180: check-documentation-videos-format] Error 1

Additional Checks

• The PR targets master for a bugfix, or next for a feature