Enhance CI

This MR is to valid / fallback changes done in the CI.
In fact I already added the only:changes rule while working on !75 (merged) , but there is still room for improvement :

• do not retest unchanged files (we can discuss it if you see a scenario where we need to force tests on unchanged files)
• remove useless $TEST_INPUT_IMAGE var
• use the "interruptible" keyword to cancel obsolete pipelines if another one is queued
• remove flake8 (pylint is already doing same and more)
• cache dependencies (only pip packages)
• unique steps for docs, manual build, webhook also for develop branch events (but master is still default docs page)
• better trigger rules (based on MR + tags, + push release (manual trigger) for branch develop or master
• use latest image (is there a otb-ubuntu-native-develop-headless:22.04 ?)

Packaging is OK. In theory we could also remove the setup.py.
Also, a develop version of the docs should be pushed when merged.
Regarding the last two points in the desc, waiting for your advice.

FYI I used the gitlab CI script editor, it's useful for syntax (and autoformat) Closes #68 (closed) #95 (closed)