Add FAQ answer about API stability & backwards compatibility (#9855)

* new blank whatsnew

* FAQ answer on API stability

* link from API docs page

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* whatsnew

* Update doc/getting-started-guide/faq.rst

Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

* use hyphen in target names

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>
Co-authored-by: Kai Mühlbauer <kai.muehlbauer@uni-bonn.de>

doc/api.rst

@@ -10,7 +10,7 @@ This page provides an auto-generated summary of xarray's API. For more details
 and examples, refer to the relevant chapters in the main part of the
 documentation.
 
-See also: :ref:`public api`
+See also: :ref:`public-api` and :ref:`api-stability`.
 
 Top-level functions
 ===================

doc/getting-started-guide/faq.rst

@@ -416,7 +416,19 @@ would certainly appreciate it. We recommend two citations.
                  url    = {https://doi.org/10.5281/zenodo.59499}
                 }
 
-.. _public api:
+.. _api-stability:
+
+How stable is Xarray's API?
+---------------------------
+
+Xarray tries very hard to maintain backwards compatibility in our :ref:`api` between released versions.
+Whilst we do occasionally make breaking changes in order to improve the library,
+we `signpost changes <https://docs.xarray.dev/en/stable/contributing.html#backwards-compatibility>`_ with ``DeprecationWarnings`` for many releases in advance.
+(An exception is bugs - whose behaviour we try to fix as soon as we notice them.)
+Our `test-driven development practices <https://docs.xarray.dev/en/stable/contributing.html#test-driven-development-code-writing>`_ helps to ensure any accidental regressions are caught.
+This philosophy applies to everything in the `public API <https://docs.xarray.dev/en/stable/getting-started-guide/faq.html#what-parts-of-xarray-are-considered-public-api>`_.
+
+.. _public-api:
 
 What parts of xarray are considered public API?
 -----------------------------------------------

doc/whats-new.rst

@@ -115,6 +115,8 @@ Documentation
 ~~~~~~~~~~~~~
 - A chapter on :ref:`internals.timecoding` is added to the internal section (:pull:`9618`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
+- Clarified xarray's policy on API stability in the FAQ. (:issue:`9854`, :pull:`9855`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Internal Changes
 ~~~~~~~~~~~~~~~~