Improve the docs of the filter() arguments

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR moves the documentation of the filter options/arguments to an own page of the docs rather that having them as part of the slice() docstring.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.0%. Comparing base (ddbb88e) to head (802f8ae).
Report is 28 commits behind head on main.

Additional details and impacted files

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##            main    #870   +/-   ##
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  Coverage   95.0%   95.0%           
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  Files         64      64           
  Lines       6134    6191   +57     
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+ Hits        5828    5884   +56     
- Misses       306     307    +1     

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[phackstock]

Looks good to me in principle. I think it would be easier for the users if you had a short example of each filter. You could use the same data frame every time and filter in different ways.

[danielhuppmann]

Thanks @phackstock - agree that we should extend the docs to include examples and I'll start an issue for it to be tackled before release 3.0, but for now, my aim was to get the docs out of core.py.

[phackstock]

Thanks for the clarification @danielhuppmann, in this case I'd say good to merge.