Generating links with `link text <./path/to/doc#anchor>`__ does not work for html.
It produces a link to `./path/to/doc#anchor` instead of `./path/to/doc.html#anchor`.
It would be tempting to add `.html` before `#` but would likely cause some more issues
when generating the documentation as pdf or epub.
References on the other hand will work regardless of the output format.
These two tests are used when checking if a new page type can be created
under a page instance, or if an existing page type can move under a page
instance.
`is_creatable` better reflects what it is used for, and stops any
confusion between Wagtail's `is_abstract` and Django's `Meta.abstract`.
`is_creatable` takes in to account `Meta.abstract` now, so developers
will no longer need to set both `is_abstract` and `Meta.abstract`.
Documentation for the new attribute has been added, as well as tests.