Use render-link hook to resolve references in specs

[chalin]

• Fixes [infra] Eliminate use of relref in spec and other pages #6025
• Adds code to the render-link hook to resolve intra-spec references in a manner that was previously done using relref. The relref function is no longer used / emitted.
• Greatly simplifies scripts/content-modules/adjust-pages.pl by removing link-transformation code that was previously used to insert relref function calls, and rename README.md. We might eventually be able to move more of the adjust-pages.pl functionality into the render-link hook.

The only change to the generated site files is the addition of trailing / at the end of /docs/specs/otel/document-status links in semconv pages:

$ (cd public && git diff -bw --ignore-blank-lines -I '/docs/specs/otel/document-status' -I 'modified_time|Last modified|dateModified' -- . ':(exclude)*.xml') | grep ^diff
diff --git a/site/index.html b/site/index.html

Preview: https://deploy-preview-6032--opentelemetry.netlify.app/docs/specs/

[tiffany76]

RSLGTM.