docs: pin a version of markedjs from cdnjs

[jbr]

Hi! It seems like the docs site is currently broken. Previous to this commit, the cdn markedjs url was not versioned, so when markedjs v13 introduced breaking changes on 2024-06-12, the rustfmt docs site automatically pulled them in.

In addition to fixing that problem by pinning to v12, this PR:

• Expands the cdnjs links and script tags to reflect their recommendations, including an integrity hash so the browser can check that the retrieved executable content is in fact as-expected.
• Switches markedjs and vue from jsdelivr to cdnjs, which should be slightly faster as the browser already will have a reusable http/tls connection to cdnjs from other resources.
• Uses the minified vue file, silencing a warning that vue was being used in development mode.

However, this PR does not change the versions of any dependencies, many of which are significantly out of date.

Thanks!

[ian-h-chamberlain]

@ytmimi thanks for the quick response, just confirming these changes do in fact resolve the problem for me (tested on Firefox 127.0 and Safari 17.5).

Seems like this change would help prevent future similar breakages from upstream too! 👍