Fixes#11938
* Add `wagtail start` to the management commands reference
* Add detail on how to creating templates with the need to escape template syntax by using the verbatim tag
- Move all generic (page/non-page) class references to the top of the documentation
- Use better heading level nesting cleanly isolate the full page report example
- Relates to #12428
- Move model reference to directly under the references section, not the reference/pages section
- Move panel reference to directly under the references section, not the reference/pages section
- Merge panel API with panels reference pages
- Fix confusing heading levels in the page model recipes page
- Fix title in model reference to align with docs style guide
- Adopt US spelling for customize (panels) reference key
Previously, translations for messages such as "1 page and 3 child pages have been published" were handled with separate branches for the "one parent page" vs "multiple parent pages" cases, to work around ngettext only being able to handle pluralisation for one item in the string. This fails for languages such as Polish where the pluralisation does not follow the straightforward one/many distinction.
Instead, we can translate "N pages" and "N child pages" separately before forming the final message. We generally avoid translating sentence fragments in isolation as this can make things difficult for translators if the sentence structure does not match the English version - but this is hopefully an improvement over the current situation, as well as simplifying the code and reducing repetition.
However, leave behind notes that mark settings/features that are deprecated but not yet removed, as this would be removing information that isn't in the main body of the text (and moving it out of a versionchanged note would make it harder to find when we come to remove it properly in 7.0).
The current version is set to 2.3 at the top level of the test. If the
test case uses 5.15.1 as the latest version, then the test should've
failed – but it didn't because we're missing a few promises to be
awaited.
To verify, try reverting the version diff and re-run the test. It would
fail with the new assertions.