Update release notes for 0.15.0

[dstansby]

[Description of PR]

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (32545ec) to head (4be42f4).
Report is 29 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #691   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files          62       62           
  Lines        2749     2749           
=======================================
  Hits         2741     2741           
  Misses          8        8           

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