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