Prepare repository for hosting multiple packages/sites

[parlough]

• Integrate packages from site-shared into repository
  • PR: Integrate packages from site-shared #13190
  • Commit: 268e2f0
• Move docs website content into a new directory that will contain multiple sites
  • PR: Nest docs website in new sites directory #13204
  • Commit:
• Update dash_site tooling to support multiple sites in a singular repository
  • PR:
  • Commit:
• Extract out some shared site behavior into a new package as a POC
  • PR:
  • Commit:
• Design and set up up new issue label system
• Set up automatic PR labeling based on modified contents
• Configure GitHub actions to be more granular to avoid unnecessary checks and builds