Migrate opentelemetry-js website_docs back here as the single source of reference

[chalin]

In #661 (comment), @dyladan wrote:

Personally, I see huge advantages of having only one place where the documentation resides.

Ok then, @dyladan shall we move the JS website_docs back into this repo? Doing so is consistent with the docs management scheme we'll be promoting moving forward. /cc @austinlparker

---

• Update the JS pages here to reflect the otel-js content of website_docs - otel-js website_docs at HEAD #722
• Submit a PR over otel-js to delete website_docs - Drop website_docs folder opentelemetry-js#2469
• Edit the JS index page to remove the custom (cascaded) fields like github_repo -- we'll be using the default values moving forward. JS: make GH page links refer to this repo #733

[chalin]

@dyladan - if you agree, then #696 can be closed.

[dyladan]

@open-telemetry/javascript-maintainers I'm ok with this if you are

[austinlparker]

I'm good with this - in the future, docs will either live in here or be linked via submodule so that there's only one copy of them at any given time.