Specification: rework of how the spec is generated, and link fix

[chalin]

• Contributes to Integrate the OTel specification into the website #906
• Rather than place generated files under contents, we now use a mount point. This is much cleaner, since we never mix generated and version-controlled files in the same directories.
• In the process, I discovered that one link was wrong in /docs/reference/specification/library-guidelines/ (see the diff below). This PR fixes the link. Other than that, there is no change to the generated site files:

  $ (cd public && git diff -- . ':(exclude)*.xml')            

  diff --git a/docs/reference/specification/library-guidelines/index.html b/docs/reference/specification/library-guidelines/index.html
  index 3f0f4801..d967e28f 100644
  --- a/docs/reference/specification/library-guidelines/index.html
  +++ b/docs/reference/specification/library-guidelines/index.html
  @@ -789,7 +789,7 @@ This document does not attempt to describe the details or functionality of the O
          <p>This document defines common principles that will help designers create OpenTelemetry clients that are easy to use, are uniform across all supported languages, yet allow enough flexibility for language-specific expressiveness.</p>
   <p>OpenTelemetry clients are expected to provide full features out of the box and allow for innovation and experimentation through extensibility.</p>
   <p>Please read the <a href="/docs/reference/specification/overview/">overview</a> first, to understand the fundamental architecture of OpenTelemetry.</p>
  -<p>This document does not attempt to describe the details or functionality of the OpenTelemetry client API. For API specs see the <a href="/docs/reference/">API specifications</a>.</p>
  +<p>This document does not attempt to describe the details or functionality of the OpenTelemetry client API. For API specs see the <a href="..">API specifications</a>.</p>
   <p><em>Note to OpenTelemetry client Authors:</em> OpenTelemetry specification, API and SDK implementation guidelines are work in progress. If you notice incomplete or missing information, contradictions, inconsistent styling and other defects please let specification writers know by creating an issue in this repository or posting in <a href="https://cloud-native.slack.com/archives/C01N7PP1THC" target="_blank" rel="noopener" class="external-link">Slack</a>. As implementors of the specification you will often have valuable insights into how the specification can be improved. The Specification SIG and members of Technical Committee highly value your opinion and welcome your feedback.</p>
   <h2 id="requirements">Requirements</h2>
   <ol>

Preview: https://deploy-preview-1152--opentelemetry.netlify.app/docs/reference/specification/library-guidelines/