Docs: remove empty how-to stubs

[atravitz]

The how-to placeholders I'm removing in this PR are ones that I consider to be less urgent, and I'd like to have our docs in a stable state without missing content:

• ComponentMapping: I think we only need AtomMapping, if anything
• AtomMapping: is this something we see developers customizing anytime soon?
• AtomMapper: is this something we see developers customizing anytime soon?

I'm open to thoughts on which, if any, of these should be high priority.

Tips

• Comment "pre-commit.ci autofix" to have pre-commit.ci atomically format your PR.
  Since this will create a commit, it is best to make this comment when you are finished with your work.

Checklist

• Added a news entry

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (e7c6688) to head (e36c805).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #715   +/-   ##
=======================================
  Coverage   98.79%   98.79%           
=======================================
  Files          40       40           
  Lines        2492     2492           
=======================================
  Hits         2462     2462           
  Misses         30       30           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

No API break detected ✅