ModelAdmin to Snippets Documentation Update

[dkirkham]

This is a continuation of #472 to address #459.
It also addresses the modeladmin to snippet transition, part of #476.

In particular, this adds documentation including initial release notes for a 4.0 release.

I believe I've found and modified anything relating to the move from ModelAdmin to Snippets, but a review looking for any missed changes would be appreciated.

TODO: in this or another PR, update the images in the documentation, which are mostly Wagtail 2 era screenshots.

[dkirkham]

New documentation screenshots have been added. This PR is now ready for review.

Note that there are two copies of the screenshots, in docs/source/_static/images (used for the docs) and in screenshots. I don't know what the latter are for, and maybe they are not required?

[MrCordeiro]

I haven't finished reading everything, this is a fantastic work so far!

Something I remembered: the translation files are probably still referring to modeladmin.py and not menuadmin.py. I think it makes sense to run the makemessages and compilemessages in this PR so that it updates the module name, what do you think?

The docs/source/settings_reference.rst also had references to ModelAdmin. Gif you get those?

[dkirkham]

I've run python manage.py makemessages -a and python manage.py compilemessages locally, and it is picking up lots of changes from previous work. It might make sense to do this change as a separate PR once a few other PRs are merged, rather than incorporating it into this or my other existing PRs.

The docs/source/settings_reference.rst references to modeladmin should have been picked up in #482.

[dkirkham]

I don't know why the pre-commit.ci is still stalled after a day - if you can see something amiss with the PR, let me know so I can fix it.

[MrCordeiro]

I've run python manage.py makemessages -a and python manage.py compilemessages locally, and it is picking up lots of changes from previous work. It might make sense to do this change as a separate PR once a few other PRs are merged, rather than incorporating it into this or my other existing PRs.

The docs/source/settings_reference.rst references to modeladmin should have been picked up in #482.

Ok on both counts. Feel free to merge it!