Convert relative site URLs in release notes

[parlough]

We no longer use {{site.url}} in Markdown files on the site to enable easier contributing, so {{site.url}} doesn't work as a replacement target anymore. Instead the site-relative image links are formatted as ![alt text](/tools/...). Same goes for other non-image links.

For DevTools to handle this case, this PR uses a regex to recognize Markdown link format to insert the docs site URL or the current debug URL. I used replaceAllMapped over lookahead/lookbehind assertions due to browser support still being limited.

[kenzieschmoll]

Thanks for the PR! Have you verified that the markdown viewer in DevTools still works as expected? (for both the local debugging case and for loading notes from the flutter website in production)? You can click the release notes button here to trigger opening the viewer:

[parlough]

Sorry about the delay, forgot to return to this. Yes, I've tried these latest changes out with a local debugging instance and the production site.