Documentation - Configure RTD to build all offline formats

- https://docs.readthedocs.io/en/stable/config-file/v2.html#formats

Fixes #12484

.readthedocs.yml

@@ -9,5 +9,6 @@ python:
       path: .
       extra_requirements:
         - docs
+formats: all
 sphinx:
   fail_on_warning: True

CHANGELOG.txt

@@ -17,6 +17,7 @@ Changelog
  * Docs: Refine titles, references and URLS to better align with the documentation style guide, including US spelling (Srishti Jaiswal)
  * Docs: Recommend a larger `DATA_UPLOAD_MAX_NUMBER_FIELDS` when integrating Wagtail into Django (Matt Westcott)
  * Docs: Improve code highlighting and formatting for Python docstrings in core models (Srishti Jaiswal)
+ * Docs: Re-enable building offline formats in online documentation (Read the docs) for EPUB/PDF/HTML downloads (Joel William)
  * Maintenance: Close open files when reading within utils/setup.py (Ataf Fazledin Ahamed)
  * Maintenance: Avoid redundant `ALLOWED_HOSTS` check in `Site.find_for_request` (Jake Howard)
 

docs/releases/6.4.md

@@ -34,6 +34,7 @@ depth: 1
  * Refine titles, references and URLS to better align with the documentation style guide, including US spelling (Srishti Jaiswal)
  * Recommend a larger `DATA_UPLOAD_MAX_NUMBER_FIELDS` when [integrating Wagtail into Django](../getting_started/integrating_into_django.md) (Matt Westcott)
  * Improve code highlighting and formatting for Python docstrings in core models (Srishti Jaiswal)
+ * Re-enable building offline formats in online documentation (Read the docs) for EPUB/PDF/HTML downloads (Joel William)
 
 ### Maintenance