docs: Versioning policy (#3488)

* docs: create `versioning.rst` placeholder

* docs: add link to `roadmap.rst`

* docs: Initial `versioning.rst` draft

Based on discussions in #3454 (comment) and #3454 (comment)

#3454

* docs: fix version number in italics

* docs: add `Version numbers` heading

* docs: add `Deprecation` section

* docs: Clarify upstream breaking -> warning option

#3488 (comment)

* docs: add undoc statement

#3488 (comment)

* docs: Clearly define **Public API**

Intentionally simple, see linked thread for discussion

#3478 (comment)

* docs: Tweak formatting of deprecation section

doc/about/roadmap.rst

@@ -13,3 +13,4 @@ The roadmap for Vega-Altair and related projects can be found in
    code_of_conduct
    governance
    citing
+   versioning

doc/about/versioning.rst

@@ -0,0 +1,46 @@
+Versioning
+==========
+Vega-Altair has historically released major versions that coincide with those of Vega-Lite_.
+
+As the projects have matured, and major versions become less frequent, there has been a growing need to introduce breaking changes between these major versions.
+Such changes would allow Vega-Altair to address technical debt and improve upon API ergonomics.
+
+To ensure future releases clearly communicate changes, Vega-Altair will be working towards adopting SemVer_.
+
+Public API
+----------
+Functionality documented in :ref:`api` defines the Vega-Altair public API.
+
+Version numbers
+---------------
+
+A Vega-Altair release number is composed of ``MAJOR.MINOR.PATCH``.
+
+* Backward incompatible API changes increment **MAJOR** version (``4.2.2`` - ``5.0.0``)
+* New backward compatible functionality increment **MINOR** version (``5.2.0`` - ``5.3.0``)
+* Backward compatible bug fixes increment **PATCH** version (``5.1.1`` - ``5.1.2``)
+
+**MAJOR** versions will *likely* continue to increase with a **MAJOR** increment to Vega-Lite_.
+
+Deprecation
+-----------
+Deprecation warnings may be introduced in **MAJOR** and **MINOR** versions, 
+but the removal of deprecated functionality will not occur until *at least* the next **MAJOR** version.
+
+For upstream breaking changes that trigger a **MAJOR** version, 
+we *may* provide a deprecation warning if we consider the change especially disruptive.
+
+Starting in version ``5.4.0``, all deprecation warnings *must* specify:
+
+* the version number they were introduced
+
+Where possible, deprecation warnings *may* specify:
+
+* an alternative function/method/parameter/class to use instead
+* an explanation for why this change had to be made
+
+Deprecated functionality *may* be removed from the Vega-Altair documentation, if there is a 
+suitable replacement and we believe inclusion of both could confuse new users.
+
+.. _Vega-Lite: https://github.com/vega/vega-lite
+.. _SemVer: https://semver.org/