Text Rework cleanup (#15887)

# Objective

Cleanup naming and docs, add missing migration guide after #15591 

All text root nodes now use `Text` (UI) / `Text2d`.
All text readers/writers use `Text<Type>Reader`/`Text<Type>Writer`
convention.

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## Migration Guide

Doubles as #15591 migration guide.

Text bundles (`TextBundle` and `Text2dBundle`) were removed in favor of
`Text` and `Text2d`.
Shared configuration fields were replaced with `TextLayout`, `TextFont`
and `TextColor` components.
Just `TextBundle`'s additional field turned into `TextNodeFlags`
component,
while `Text2dBundle`'s additional fields turned into `TextBounds` and
`Anchor` components.

Text sections were removed in favor of hierarchy-based approach.
For root text entities with `Text` or `Text2d` components, child
entities with `TextSpan` will act as additional text sections.
To still access text spans by index, use the new `TextUiReader`,
`Text2dReader` and `TextUiWriter`, `Text2dWriter` system parameters.