Improve TextSpan docs

[rparrett]

Objective

Our TextSpan docs include a code example that does not actually "work." The code silently does not render anything, and the Text*Writer helpers fail.

This seems to be by design, because we can't use Text or Text2d from bevy_ui or bevy_sprite within docs in bevy_text. (Correct me if I am wrong)

I have seen multiple users confused by these docs.

Also fixes #16794

Solution

Remove the code example from TextSpan, and instead encourage users to seek docs on Text or Text2d.

Add examples with nested TextSpans in those areas.