Retire and archive `dart-lang/site-shared`

[parlough]

• Move still necessary content out of repository.
• Add notes to READMEs and other docs in the repository indicating the status and any new locations. (Prepare repository to be retired and archived dart-archive/site-shared#265)
• Archive repository and/or move to dart-archive org

[parlough]

We've now removed our dependencies on dart-lang/site-shared, moved what's still used, and added notes about archival across the repository.

@athomas Could you (or someone else) help with archiving the dart-lang/site-shared repository and moving it to https://github.com/dart-archive? Low priority but just wanted to note that it's no longer used. Let me know if you need any further information to do so. Thanks!

[athomas]

Done

[parlough]

Thanks @athomas! I appreciate the help :)