Allow docs preview comments on PRs

[cncastillo]

Summary

• give the Docs Preview workflow pull-requests: write so it can create/update PR comments
• serialize docs deploy and docs preview cleanup with one gh-pages-deploy concurrency group

Root cause

The preview comment workflow found PR #769 and computed the preview URL, but GitHub rejected POST /issues/769/comments with 403 Resource not accessible by integration. The response advertised accepted permissions as issues=write; pull_requests=write; the workflow only had pull-requests: read.

The cleanup workflow can also push gh-pages while the normal Documentation job is still building, which causes git push ... HEAD:gh-pages to fail with fetch first. Both gh-pages writers now share the same concurrency group.

Testing

• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].each { |f| YAML.load_file(f) }; puts "yaml ok"'\n- git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.68%. Comparing base (87f621f) to head (e27f751).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
- 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)

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see 8 files with indirect coverage changes

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