Fix Bonsai link in Elasticsearch docs

The search docs [0] currently link to a Bonsai signup page that 404s [1].
The signup page is currently a different page [2].
This commit replaces the Bonsai link with their homepage [3],
to avoid being overly dependent on their URL structure.

[0] https://docs.wagtail.io/en/v2.11.2/topics/search/backends.html
[1] https://bonsai.io/signup
[2] https://app.bonsai.io/signup
[3] https://bonsai.io/

docs/topics/search/backends.rst

@@ -180,7 +180,7 @@ If you prefer not to run an Elasticsearch server in development or production, t
 -  Run ``./manage.py update_index``
 
 .. _elasticsearch-py: https://elasticsearch-py.readthedocs.org
-.. _Bonsai: https://bonsai.io/signup
+.. _Bonsai: https://bonsai.io/
 
 Amazon AWS Elasticsearch
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