Implement all docutils admonitions + handle admonition arguments correctly

[wouterj]

We currently do not support all admonitions, given it is easy to add support (by extending AbstractAdmonitionDirective) I would say we should add support for it.

I've created a branch that updates the tests, which can be used as a base when working on this issue: main...wouterj:guides:admonitions

In here you can also find some other admonition related tests that need to be fixed:

• The generic .. admonition:: directives behaves different from all note/warning/caution/hint/etc directives concerning it's argument.
  For all admonitions, the text directly following the directive is considered the first paragraph of a directive: d0cc00a
  For the generic admonition, this "argument" is treated as the title and is also used to generate a class name: 9be2145
• The version changeset directives (versionadded/versionchanged/deprecated) are not implemented as admonitions but are their own "VersionChange" directive (with a related VersionChangeNode). They are rendered a bit differently and require an argument (the changed version): 1f54b62