Add the specification to the docs

[chalin]

Here's a preview of the spec published via the website:

• https://deploy-preview-866--opentelemetry.netlify.app/docs/reference/specification/

This requires no extra effort on the part of the spec maintainers other than to let us know when the spec is updated so that we can update it here too.

Contributes to #852. (There will be some followup & cleanup items, but they can be addressed in a separate PR.)

@bhs @tedsuo @open-telemetry/specs-approvers WDYT?

/cc @celestehorgan @nate-double-u

[reyang]

Here's a preview of the spec published via the website:

• https://deploy-preview-866--opentelemetry.netlify.app/docs/reference//specification/

How do I access the link? It shows HTTP 404.

[chalin]

There's a build issue on Netlify that doesn't occur locally. I'm still investigating. I'll comment here once the issue is resolved.

[chalin]

The preview is now live! ✨

[chalin]

Repo links (like Edit page), now refer to the https://github.com/open-telemetry/opentelemetry-specification repo.

(All such links work, except for the top-level Specification index page, since that page isn't in the website repo yet -- the page is being added by this PR.)

[chalin]

As mentioned in #852 (comment):

• I've implemented option 3 [put the spec under /docs/reference] , and
• Replaced the "Edit page" button by a "View page source"

PTAL

[austinlparker]

Hold merge until after 11/9/21

[chalin]

With that in mind, unless there are other blockers we'll be merging #866 next Tuesday.

No objections raised. Merging.