Ensure non-API docs are hosted on opentelemetry.io

[chalin]

As a part of improving user experience and SEO, all OTel docs should be hosted on opentelemetry.io possibly aside from low-level technical notes and autogenerated docs (like API references) that require language-specific tools.

The initial notes below come from the items marked as "(0)" in #730:

• Publish the specification via opentelemetry.io #852
• Collector: consider migrating relevant parts of the Collector docs folder
• Community pages:
  • Host roadmap on website #2167
  • Host Mission page on website from community repo #1289
• C++: has a docs folder containing .md files and some RST documentation (under public) published at opentelemetry-cpp.readthedocs.io.
• Go: Move Go and Ruby docs into this repo #1935
• Java:
  • Move Java docs into this repo #989
  • Java instrumentation:
    • Migrate relevant docs to opentelemetry.io opentelemetry-java-instrumentation#4523
      • Migrate Java Agent doc pages to this repo, starting with Agent config #1229
    • Why two manual instrumentation pages? opentelemetry-java-instrumentation#4524
      • Added @WithSpan documentation for Java #1178
      • Drop manual-instrumentation and redirect to otel website pages opentelemetry-java-instrumentation#5516
    • Remove documentation captured on website from READMEs opentelemetry-java-instrumentation#1580
• .Net/C# - has a docs folder containing .md files.
  • Moving docs to opentelemetry.io opentelemetry-dotnet#2876
• PHP:
  • Migrate PHP website_docs back to this repo #1154
  • Move website_docs back to website repo? (Decide by Oct 1) opentelemetry-php#423 (comment)
  • Migrate integration pages from PHP repo docs folder? #1157
  • Documentation for opentelemetry.io opentelemetry-php#405
• Python: the Python docs (from docs directory published via Read the Docs) seem extensive. Migration is in progress:
  • Migrate Python docs to this repo #1109
    • Non-API docs should be hosted on the website only opentelemetry-python#2172
  • Docs changes to split out automatic instrumentation, instrumentation packages, and manual instrumentation opentelemetry-python#2319
• Ruby: Migrate Ruby docs to this repo #1942
• Rust: see Rust docs

[Petrie]

Why is there no golang in the task list. @chalin

[cartermp]

I won't speak for @chalin, but I'd eventually love to get the golang docs in here too. Just haven't gotten around to it, and it hasn't been difficult to update them in the current arrangement. Will bring it up in the next comms SIG. Go and Ruby are the only remaining repos that we submodule docs from.