Add many PrivateApi attributes to hide unimportant stuff in docs

[iJungleboy]

The docs should help people use the Oqtane API as best as possible. As such, we should hide objects which are just for internal work or won't be used as an API. I've added [PrivateApi] attributes to ca. 20 classes which clutter up the docs without real benefit to the user.

As an example, I've removed tagged all the HtmlText classes, since that's not an API I expect others to need in the docs - this removes these entries from the docs:

• a few others, all which I'm convinced serve no benefit in the docs.

Otherwise no code changes.