CI: check-links: enable caching and check for warnings

[chalin]

• Contributes to CI: add GH action to surface build warnings and other secondary checks #2814
• Enable simple node-based caching
• Separates concerns. We now have separate jobs for:
  • Building the site and running the link checker
  • Checking for required refcache updates
  • Checking for build issues (including warnings about use of external links when a local path should be used)
• I've disabled the failure of the BUILD WARNINGS job until Adjust to move Semantic Conventions out of OTel spec #2721 is fully resolved

https://github.com/open-telemetry/opentelemetry.io/actions/workflows/check-links.yml used to take almost 3 minutes, it is now down to 1 min 40 sec. Update: it's now 2 min 15 sec on a rerun; I couldn't get it to work using a partial install. Not as much an improvement as I would have hoped, but it is an improvement. I could probably trim devDependencies, but this will do for now.

[svrnm]

open-telemetry/opentelemetry.io/actions/workflows/check-links.yml used to take almost 3 minutes, it is now down to 1 min 40 sec. Update: it's now 2 min 15 sec on a rerun; I couldn't get it to work using a partial install. Not as much an improvement as I would have hoped, but it is an improvement. I could probably trim devDependencies, but this will do for now.

every minute counts :-) it looks like checking the links itself is taking ~50s, another 50s go in npm install (added 2236 packages, and audited 2237 packages in 53s)

[svrnm]

@chalin just FYI, I ran this tool today to see which packages have the most dependencies. The biggest contributor are netlify-cli (1336), textlint & deps (696) gulp (482), cspell (183), so if you want to tune the job in a way that it only installs the packages needed, you might get some time back.

[chalin]

I tried that already, see 6c02f16, but it didn't save us any time as far as I could tell. If you think that it might still be helpful to try, I can roll back to that commit. WDYT?

[chalin]

Ok, I'm going to try the reduced dependencies list again.

[chalin]

Using a reduced-dependencies build worked! The problem I faced before was that my jq query wasn't working. The slowest action is now slightly below 2 min.

[chalin]

Seeded refcache change and the action successfully detected it.