Document the Global Styles Presets

[youknowriad]

Blocked by #35619

This PR adds initial documentation for the global styles presets feature (for block themes).

While working on this, I noticed a number of docs that look similar or duplicate each other, I wonder if it's intentional and whether we need to consolidate at some point:

• Block Themes documentation and folder structure is duplicated in two files.
• Theme.json config docs (settings and styles...) are also duplicated, theme theme-json.md file and the create-block-theme.md file.

[carolinan]

We are moving what's "stable" in 5.9 to the theme developer handbook https://developer.wordpress.org/themes/block-themes/
so these original documents can absolutely be "cleaned up". I don't think they can be removed since there may be many external, incoming links.

[ryanwelcher]

The docs look good to me. Just to confirm that this feature doesn't currently work, correct? I was not able to get it working using the docs as a reference.

[ntsekouras]

LGTM, thanks Riad!

[youknowriad]

@ryanwelcher yes, it only works in the linked PR #35619