Text Rework cleanup

[MiniaczQ]

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.

[MiniaczQ]

I'm also thinking whether this should reintroduce deprecated Text2dBundle and TextBundle so people get a deprecation message with migration instructions
@alice-i-cecile

[alice-i-cecile]

I really like most of these changes :) If we revert the TextUi change IMO it's uncontroversial.

[alice-i-cecile]

I'm also thinking whether this should reintroduce deprecated Text2dBundle and TextBundle so people get a deprecation message with migration instructions @alice-i-cecile

Yes please!

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[ickshonpe]

Seems fine. I'd prefer if TextUiReader/Writer was just called TextReader/Writer but then we'd need to rename the underlying type. It's not worth fussing over for now.