Update README.md disclaimer wording and formatting

[colorful-tones]

I've wrapped the Disclaimer in the README with markdown [!IMPORTANT] to highlight the callout for new visitors and updated the description to try to streamline understanding of potential damage.

We could consider utilizing [!NOTE], [!TIP], [!WARNING], or [!CAUTION] instead. See previews of each here: https://github.com/orgs/community/discussions/16925

Here is the link to the rich diff for previewing these changes (although the purple coloring of the [!IMPORTANT] is not rendered in the preview): https://github.com/WordPress/create-block-theme/pull/703/files?short_path=b335630#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5

[mikachan]

These changes look good to me, thank you! I like the use of [!IMPORTANT].

Should we make a similar update to the readme.txt file as well? As that's the text used on the wp.org plugin directory page.

[colorful-tones]

Should we make a similar update to the readme.txt file as well? As that's the text used on the wp.org plugin directory page.

I added the updated disclaimer in the plugin's readme.txt as well.

[matiasbenedetto]

LGTM