Add semantic-conventions as a submodule and get schemas from there, with one exception

[chalin]

• Contributes to Adjust to move Semantic Conventions out of OTel spec #2721
  • Brings in https://github.com/open-telemetry/semantic-conventions as a content-modules
• Closes Schema files should now be served from the Semantic Conventions repository #2907
  • Also ensure that we temporarily get the 1.21.0 schema from the OTel spec repo (will track removal of this special case via 2721)

There are no changes in the generated site files:

$ npm run build
...
$ (cd public && git diff) # no change
$