Document the sub-parts of Textual builtin compound widgets, from a styling point of view

[davep]

Some of our widgets, Markdown and Select being two good examples, are compound widgets with sub-parts that have style, which developers would wish to style, but which aren't spoken about in the documentation. In the case of Markdown it's obvious that there are sub-parts (headings, lists, etc) that could be styled but it's not obvious what they're called. In the case of Select it's not obvious that there is a SelectCurrent that is involved in showing focus, etc...

Given that there's no automated method of exposing these, and even if there was there's no good way of automatically documenting why and how they're used, we should extend the documentation for such widgets to include styling guidelines.

[willmcgugan]

Will consider this on a widget by widget basis. Closing for now..