chore: skip npmjs.com in linkchecking while it returns 403 to non-browsers

[trentm]

(#5839 (comment) for background.)

npmjs.com is returning 403s for link checks. This breaks linting (in the npm run docs:test step). For example:

% curl -I https://www.npmjs.com/
HTTP/2 403
date: Fri, 19 Sep 2025 18:27:54 GMT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (a382f06) to head (377ddd7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5948   +/-   ##
=======================================
  Coverage   95.07%   95.07%           
=======================================
  Files         308      308           
  Lines        8037     8037           
  Branches     1626     1626           
=======================================
  Hits         7641     7641           
  Misses        396      396           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[trentm]

As expected, the Lint workflow fails (as it is doing with other PRs today):

https://github.com/open-telemetry/opentelemetry-js/actions/runs/17866685582/job/50810583450?pr=5948

> [email protected] docs:test
> linkinator docs --silent --retry && linkinator doc/*.md --skip http://localhost:3000 --skip http://localhost:9464 --skip https://github.com/ --silent --retry --directory-listing

🏊‍♂️ crawling docs
🤖 Successfully scanned 51 links in 0.328 seconds.
🏊‍♂️ crawling doc/context.md doc/esm-support.md doc/exporter-guide.md doc/frequently-asked-questions.md doc/instrumentation-guide.md doc/metrics.md doc/propagation.md doc/sdk-registration.md doc/tracing.md doc/upgrade-to-2.x.md
[403] https://www.npmjs.com/package/@opentelemetry/exporter-metrics-otlp-grpc
[403] https://www.npmjs.com/package/@opentelemetry/exporter-metrics-otlp-proto
doc/metrics.md
  [403] https://www.npmjs.com/package/@opentelemetry/exporter-metrics-otlp-grpc
  [403] https://www.npmjs.com/package/@opentelemetry/exporter-metrics-otlp-proto
ERROR: Detected 2 broken links. Scanned 36 links in 2.285 seconds.

[trentm]

I've asked in npm discussions if this is intentional, FWIW: https://github.com/orgs/community/discussions/174098

Not sure if that is a reasonable forum. https://www.npmjs.com/support says:

If you have suggestions for how we can improve npm please open a discussion in our feedback forum.

[maryliag]

thanks for fixing this, it was annoying 😄