Migrate demo docs to this repo

[chalin]

Originally posted by @chalin in #2173 (comment):

If we agree to move the demo docs in one go, then we / @open-telemetry/demo-approvers should probably:

• Merge all pending doc PRs in the demo repo
• Agree on a demo-repo docs freeze date - 2023/02/03?
• Move the demo docs here
  • Add demo docs, temporarily via git submodule #2246
  • Add demo doc pages, drop tmp git submodule #2270
• Drop docs folder and adjust links opentelemetry-demo#730
  • Drop docs from demo repo
  • Adjust relevant demo repo pages to refer to the docs published on the website
• Declare the demo-docs freeze as over (We never officially frozen them :))

[chalin]

For reference, here is a link to the docs folder before being dropped (in case we need to compare):

• https://github.com/open-telemetry/opentelemetry-demo/tree/4b8bb4554b8888c712349d26679cca109f4c575e/docs