docs: document tox builds

[medubelko]

• Remove snapcraft_legacy as an autobuild target, since it was blocking.

• Add temporary documentation to HACKING.md, since it likely won't remain after we impose README standards.

• Have you followed the guidelines for contributing?

• Have you signed the CLA?

• Have you successfully run tox run -m lint?

• Have you successfully run tox run -e test-py310? (supported versions: py39, py310, py311, py312)

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I got tired of checking my notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.60%. Comparing base (654871d) to head (b21d514).
Report is 655 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (654871d) and HEAD (b21d514). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (654871d)	HEAD (b21d514)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5033      +/-   ##
==========================================
- Coverage   94.88%   89.60%   -5.28%     
==========================================
  Files         658      338     -320     
  Lines       55189    22345   -32844     
==========================================
- Hits        52364    20023   -32341     
+ Misses       2825     2322     -503     

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