Update theme.json documentation with fontStyles, fontWeights, fontDecorations, fontTransforms presets

[ErikBernskiold]

When updating an experimental theme project from Gutenberg 9.5 to Gutenberg 9.7, I noticed that all our font weights were suddenly missing. We were relying on the presets in experimental-theme.json.

After reviewing changelogs and issues closely, I think I've managed to find the PR where this was removed (#27555).

The documentation (https://developer.wordpress.org/block-editor/developers/themes/theme-json/) needs to be updated to reflect that these are no longer available.

A side note from the POV of an ecosystem developer:

These kind of changes are a big deal when following along with experimental theme projects, and projects extending upon Gutenberg. It would be nice, in general, if they get a little bit more attention in releases. It's not the small, trivial change that the PR makes it out to be.