Link to OTel Java website_docs via submodule

[chalin]

• Contributes to:
  • Bring in language docs via submodules #661
  • Ensure that all doc pages have a single source repo #730
• No changes to the generated site files, aside from date/time stamps:

  $ cd public && git diff -I '^<meta \w+="[^"]*(date|time)' -I '>Last modified' -- . ':(exclude)*.xml'
  $

Preview: https://deploy-preview-739--opentelemetry.netlify.app/docs/java

---

This is roughly how the PR was created:

$ rm -Rf content/en/docs/java
$ git submodule add https://github.com/open-telemetry/opentelemetry-java content-modules/opentelemetry-java
$ (cd content/en/docs && ln -s ../../../content-modules/opentelemetry-java/website_docs java)

We could use a mount point, but this works just as well.