Improve our filter documentation

Description

This MR aims to improve our filter documentation.

The patch ensures that all improvements will be seen at: https://docs.buildstream.build/elements/filter.html which I believe to be the most sensible place to host such changes.

Closes #278 (closed)

added 6 commits

• b87a7361...564cb245 - 4 commits from branch master
• 0ead1f1c - filter.py/filter.yaml: Documentation improvements
• 3c860c34 - filter.py: Add an example to the documentation

Compare with previous version

resolved all discussions

added 18 commits

• 3c860c34...7256bb0c - 16 commits from branch master
• 4ae91c15 - filter.py/filter.yaml: Documentation improvements
• 64d8af6a - filter.py: Add an example to the documentation

Compare with previous version

added 2 commits

• 0ead1f1c - filter.py/filter.yaml: Documentation improvements
• 3c860c34 - filter.py: Add an example to the documentation

Compare with previous version

added 18 commits

• 3c860c34...7256bb0c - 16 commits from branch master
• 44f72f1a - filter.py/filter.yaml: Documentation improvements
• eaae5bed - filter.py: Add an example to the documentation

Compare with previous version

added 11 commits

• eaae5bed...2b38aabe - 9 commits from branch master
• dbb3d232 - filter.py/filter.yaml: Documentation improvements
• 7e4205cb - filter.py: Add an example to the documentation

Compare with previous version

enabled an automatic merge when the pipeline for 7e4205cb succeeds

merged

mentioned in commit 4109a34a