docs: add a deprecation policy to the contributing guide

[maxrjones]

Closes #2924.

This PR adds a deprecation policy to the contributing guide, so there's a shared framework all contributors and maintainers can apply rather than re-deriving one for each PR. Making it a public doc also gives users visibility into how we handle breaking changes.

The policy is consistent with our EffVer versioning and covers scope, decision framework, mechanics, minimum period, and exceptions to the policy.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (e29ddd2) to head (153117c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4051   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          90       90           
  Lines       11981    11981           
=======================================
  Hits        11203    11203           
  Misses        778      778           

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