feat: add a @userschema/registration endpoint

[erral]

Fixes #1873

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📚 Documentation preview 📚: https://plonerestapi--1874.org.readthedocs.build/

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[erral]

@jenkins-plone-org please run jobs

[stevepiercy]

I reviewed only the docs and news, and briefly scanned the code. Per @davisagli's comment, some refactoring of the names of things in the code is needed. I'll review that after refactor, instead of flooding a review with dozens of line-by-line suggestions.

[erral]

@jenkins-plone-org please run jobs

[stevepiercy]

Some more refactoring to put username before registration, and a sprinkling of MyST on the change log. Then I think it's good to go.

I'd like a technical review from one of our colleagues, too. I'm not yet confident enough with this package.

[stevepiercy]

Docs LGTM!

https://plonerestapi--1874.org.readthedocs.build/en/1874/endpoints/userschema.html#get-the-registration-form

Let's get a technical review, too.

[erral]

@jenkins-plone-org please run jobs

[davisagli]

LGTM. Thanks @erral!

[davisagli]

@robgietema FYI, we are adding a variation of the @userschema endpoint with a different set of fields for the registration form.