DOC: Section on RTD version switcher (#1280)

* DOC: Section on RTD version switcher

* Apply suggestions from code review

* remove hash

---------

Co-authored-by: Vecko <[email protected]>
Co-authored-by: Daniel McCloy <[email protected]>

Author: VeckoTheGecko

docs/user_guide/readthedocs.md

@@ -2,6 +2,16 @@
 
 This theme comes with support for {{ rtd }}, a popular service for hosting documentation in the scientific Python community.
 
+## Version switcher
+
+Projects hosted on {{ rtd }} can use the {{ rtd }} supplied version switcher instead of the [version switcher that this theme provides](version-dropdown.rst).
+Its presence will be automatically detected by this theme, and placed in the `rtd-footer-container` node inside the primary sidebar.
+
+```{warning}
+The {{ rtd }} version switcher will be hidden any time the primary sidebar is hidden (see [this section](layout-sidebar-primary) for discussion of when the primary sidebar might get hidden automatically and how to hide it purposely).
+We intend to make {{ rtd }} switcher placement more flexible; you can track progress toward that in [this issue](https://github.com/pydata/pydata-sphinx-theme/issues/705).
+```
+
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