Docs: Clarify the status of the wp-block-styles theme support, and its intent

[andrewserong]

What?

Update the docs for the wp-block-styles theme support to clarify its intent to be used in classic themes, and that block themes should be adding styles to theme.json instead of depending on the opinionated styles of this theme support feature.

Why?

Thanks to @simison for flagging that the docs here don't nudge anyone toward baking desired styles in to theme.json for block themes. While it might be easier for themes to use this theme support, they're not guaranteed to work well with block themes or to avoid styling conflicts with rules from global styles.

How?

Update the docs hosted at: https://developer.wordpress.org/block-editor/how-to-guides/themes/theme-support/#default-block-styles

Testing Instructions

Does this read okay, and is the information accurate? I've based that changes here on discussions from @carolinan and @tellthemachines, in particular on #57841 (comment), but it'd be good to double-check that I got the overall idea right, as I could very well be missing some context about this theme support!

Testing Instructions for Keyboard

Screenshots or screencast

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Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: carolinan <poena@git.wordpress.org>

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[carolinan]

Can there be a new subheading above this section, because it can still read as if
"Without these rules, the block would result in a broken layout containing no columns at all." .... applies to wp-block-styles.

[andrewserong]

Oh, great point @carolinan! I've added an Opinionated block styles heading. Happy to update any of the wording, of course!

[ramonjd]

Nice

[andrewserong]

Thanks for the review @ramonjd! This PR isn't urgent, so I'll leave it overnight in case there's any other feedback, and then will merge it in tomorrow.