[PAGE ISSUE]: Post-publishing work for 'Concurrency in Dart'

[kwalrath]

Page URL

https://dart.dev/guides/language/concurrency/index

Page source

https://github.com/dart-lang/site-www/tree/master/src/_guides/language/concurrency/index.md

Describe the problem

Definitely:

• [x] Convert code snippets to use code excerpts. (@parlough)
• [x] Add this page to the sidenav. Link to it appropriately. (https://github.com/dart-lang/site-www/pull/3850)
• [ ] Figure out how to save an editable version of the source that has the right fonts.

Maybe:

• [ ] Change diagrams to mention port/ReceivePort/SendPort.
• [ ] Add a figure (or code?) high up in the doc to make it more inviting, and to make clear that it isn't just words.
• [ ] Give more information about performance, perhaps in its own page. https://github.com/dart-lang/site-www/pull/3696#discussion_r762007443 has some details. So does the 2.15 blog post.
• [ ] Add a new macro & style for "Flutter note:" asides?
• [ ] Add a something to the page (and/or a FAQ entry) with the term single-threaded language. It might also say Dart code executes in a predictable sequence that can’t be interrupted by other Dart code.

Expected fix

No response

Additional context

Followup to #3696.