Add content type intro and FTI reference

[davisagli]

Beginning of work on #1303

This has an intro adapted from https://github.com/plone/training/blob/main/docs/mastering-plone/dexterity.md (but trying to avoid tutorial-style content)

And a reference for the properties that can be set on an FTI to define a content type, inspired by https://github.com/plone/documentation/blob/6.0/plone.app.dexterity/schema-driven-types.md but mostly written from scratch to make sure we're covering all the properties.

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[stevepiercy]

Mostly MyST syntax changes, and a few style changes.

[davisagli]

@stevepiercy The changes to use {doc} links don't seem to be correct:

/Users/davisagli/Plone/documentation/docs/backend/content-types/index.md:154: WARNING: unknown document: portal-actions
/Users/davisagli/Plone/documentation/docs/backend/content-types/index.md:167: WARNING: unknown document: behaviors
/Users/davisagli/Plone/documentation/docs/backend/content-types/index.md:221: WARNING: unknown document: icons

[stevepiercy]

@stevepiercy The changes to use {doc} links don't seem to be correct:

/Users/davisagli/Plone/documentation/docs/backend/content-types/index.md:154: WARNING: unknown document: portal-actions
/Users/davisagli/Plone/documentation/docs/backend/content-types/index.md:167: WARNING: unknown document: behaviors
/Users/davisagli/Plone/documentation/docs/backend/content-types/index.md:221: WARNING: unknown document: icons

Ah, the path needs to be adjusted, as {doc} resolves to the current directory. I typically prefer using root-relative paths to disambiguate.

[stevepiercy]

The todos in definitions are also messed up. That might be because of a nested backtick fence from the parent glossary, but should be resolved when conversion is complete.

[stevepiercy]

Just one stray space, and still wonky todos (see updated comment), and it's good to go.

Yeah, there should be only one glossary. Lists are lists. There is slightly different HTML which can be used for CSS, <dl class="glossary"> versus <dl class="simple myst">.

[stevepiercy]

I had a 🛌 and ☕ , and noticed a couple more things. What do you think?

[stevepiercy]

FTR, I don't know what I missed on the first pass, but this test works for me:

`view_methods2`
:   ```{todo}
    some todo
    ```

We did away with the TODO, but now we have a working example.

[stevepiercy]

@davisagli just a heads up to pull my commit a5647c3 before pushing additional work.

[stevepiercy]

@davisagli I'll let you merge, assuming you might want to get another sprinter to look it over.

[davisagli]

@stevepiercy Jens already reviewed it, so I'll go ahead.