Harden docs preview workflows

[cncastillo]

Problem: the previous gh-pages concurrency lock was on the whole Documentation job. GitHub only keeps one pending job per concurrency group, so concurrent docs runs could cancel pending docs jobs instead of queueing them. The preview comment workflow also waited for the entire CI workflow to succeed, even though the relevant signal is the Documentation job.\n\nSolution: remove the broad docs/cleanup concurrency lock, and make the preview comment workflow check that the current PR head has a successful Documentation job before posting/updating the link.\n\nValidation: parsed the touched workflow YAML, ran git diff --check, and ran actionlint on CI.yml, docs-preview.yml, and docs-preview-cleanup.yml.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.68%. Comparing base (d3b13a1) to head (e1ae6f5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
- Coverage   90.01%   88.68%   -1.34%     
==========================================
  Files          74       74              
  Lines        5179     5171       -8     
==========================================
- Hits         4662     4586      -76     
- Misses        517      585      +68     

Flag	Coverage Δ
base	86.48% <ø> (ø)
core	80.47% <ø> (-10.17%)	⬇️
files	95.26% <ø> (ø)
komamri	88.23% <ø> (ø)
plots	90.88% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 8 files with indirect coverage changes

🚀 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.