Mark API v1 documentation as deprecated (#3429)

docs/advanced_topics/api/index.rst

@@ -11,7 +11,7 @@ There are currently two versions of the API available: v1 and v2. Both versions
 are "stable" so it is recommended to use v2. V1 is only provided for backwards
 compatibility and will be removed from Wagtail soon.
 
-See `RFC 8: Wagtail API <https://github.com/wagtail/wagtail-rfcs/blob/master/accepted/008-wagtail-api.rst#id4>`_
+See `RFC 8: Wagtail API <https://github.com/wagtail/rfcs/blob/master/accepted/008-wagtail-api.md#12---stable-and-unstable-versions>`_
 for full details on our stabilisation policy.
 
 Version 2 (recommended)

docs/reference/contrib/api/index.rst

@@ -1,5 +1,9 @@
-Wagtail API
-===========
+Wagtail API v1
+==============
+
+.. warning::
+
+    This documentation covers the deprecated version 1 of the API, which is provided for backwards compatibility only. For the current version, see :doc:`/advanced_topics/api/index`
 
 The ``wagtailapi`` module can be used to create a read-only, JSON-based API for public Wagtail content.