feat: MkDocs baseline (experiment)

[DaveSkender]

Caution

Experiment only (do not merge)

done when

• Add baseline MkDocs artifacts and configurations
  • install using Material for MkDocs
• Migrate all existing content from Jekyll format to MkDocs format
  • in-kind .yml and {% conditionals %} stuff
• combine to single stockindicators.dev site
  • with inline language tabs as separate component subfiles
• Remove all Jekyll documentation site artifacts and configurations

if accepted

• Update all corresponding GitHub Actions, including secrets, variables, hosts
  • update redirects accordingly (based on old site maps)
  • remove subdomain sites and related DNS configs

Closes #1297