More work on the monthly update post

[lqd]

Here's the bunch of fixes I mentioned when reviewing the other PRs for #297:

• pluralization of the update counts
• compute the goal counts
• use the date for the updates instead of the full timestamp
• sort the goal updates
• some visual separation between flagship goals (and goals that need help): whenever there's text, markdown visually lumps together the updates <details> with the previous goal -- mostly a matter of taste, we can tweak/remove it if you don't like it. This weirdness may also appear when goal updates have tl;drs as well...
• cleanups to ignore: warnings, unused files (for the llm, we can probably merge/remove a bunch of crates soon), rustfmt everything

You can check the outcome of these changes here.

Rendered