markdown cleanup (bookmarks.md, contributing.md)

[jabielecki]

Previous markdown did not produce any appealing effect on github.com renderer.
Change markdown to the style supported by github.com.

The notes were of varying importance. Using a quotation-style note where the distraction is not warranted.

Use "[!TIP]" formatting per Github formatting recommendation.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[ilyagr]

We can't really review this until you sign the CLA.

Also, we just simplified previewing the website on your machine. Can you follow https://martinvonz.github.io/jj/prerelease/contributing/#previewing-the-html-documentation and double-check that your changes don't make the website look worse?