Fix OTel spec links to page source

[chalin]

Some spec page links to GitHub page source were broken by #2677. This fixes those links and brings in a bit more code in prep for hosting the OTLP spec pages.

This PR impacts only the OTel spec pages and only the GH link URLs.

$ cd public
$ g diff | grep ^diff | grep -v 'docs/spec' # only spec pages are affected
$ g diff | grep ^diff | wc -l                                   
      96
$ g diff | grep ^diff | grep -E '^diff.*index.html$' | wc -l    # only index pages are affected
      96
$ g diff -I 'View page source|Edit this page| Create child page' # only the GH links are affected
$

Contributes to:

• Adjust to move of OTLP specs into opentelemetry-proto #2642

/cc @tigrannajaryan