docs: [FC-0074] add docs about creating new filters with pipeline steps

[mariajgrimaldi]

Description

Update how-to documents to create and implement pipeline steps considering design suggestions introduced in #240.

Here are the main documents to review:

• How to Create a New Filter with Long-Term Support: https://github.com/openedx/openedx-filters/pull/242/files#diff-2825244e325626bcad68d204d59aa3e6ff696d1b5043fd4b990bd77e88dbb9a1
• Implement Pipeline Steps: https://github.com/openedx/openedx-filters/pull/242/files#diff-f1e167403e1377f568c3a2641fb3a151cd61d26887b3a468f91669d4494aee61

Testing instructions

You can review the docs here: https://docsopenedxorg--242.org.readthedocs.build/projects/openedx-filters/en/242/how-tos/index.html

Deadline

None

Checklists

Check off if complete or not applicable:

Merge Checklist:

• All reviewers approved
• Reviewer tested the code following the testing instructions
• CI build is green
• Version bumped
• Changelog record added with short description of the change and current date
• Documentation updated (not only docstrings)
• Code dependencies reviewed
• Fixup commits are squashed away
• Unit tests added/updated
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

Post Merge:

• Create a tag
• Create a release on GitHub
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)
• Upgrade the package in the Open edX platform requirements (if applicable)

[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi!

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