docs: parallelize Read the Docs HTML build

[njzjz-bot]

Summary

• Override only the Read the Docs HTML build command.
• Run Sphinx with two workers via -j 2.
• Preserve the existing HTML builder, language, doctree directory, source directory, and output directory.
• Keep this change independent from the Linkcode experiment.

Performance measurement

This PR intentionally contains only the Sphinx worker-count change so its Read the Docs preview measures parallelism independently.

Baseline Read the Docs build 33680413:

• total build: 1889 seconds
• dependency installation: 303 seconds
• Sphinx HTML: 1524 seconds
• Sphinx worker count: 1

Both performance PRs use upstream commit 83e7245646185a2de8f7d6277edc4f64b4f500f0 as their base.

Testing

• Parsed .readthedocs.yml with PyYAML
• Ran the configured Sphinx arguments against a Sphinx 9.0.4 smoke project with -j 2
• ruff check .
• ruff format .
• Repository commit hooks, including YAML validation and Prettier

Coding agent: Codex
Codex version: codex-cli 0.144.6
Model: gpt-5.6-sol
Reasoning effort: xhigh

Summary by CodeRabbit

• Documentation
  • Improved Read the Docs HTML builds with explicit Sphinx settings.
  • Standardized English-language output and build directories.
  • Limited documentation builds to two workers for more predictable performance.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: b0811fff-fac2-4366-bc43-6956763d9821

📥 Commits

Reviewing files that changed from the base of the PR and between 83e7245 and 4cf6179.

📒 Files selected for processing (1)

• .readthedocs.yml

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📝 Walkthrough

Walkthrough

The Read the Docs configuration adds an explicit Sphinx HTML build command using two workers, English language settings, doc as the source, doc/_build/doctrees for doctrees, and $READTHEDOCS_OUTPUT/html for output.

Changes

Read the Docs build

Layer / File(s)	Summary
Configure the Sphinx HTML build .readthedocs.yml	Adds explicit Sphinx options for concurrency, language, doctree storage, source input, and HTML output paths.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related PRs

• deepmodeling/deepmd-kit#5710: Both changes update .readthedocs.yml documentation build outputs.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: parallelizing the Read the Docs HTML build.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.81%. Comparing base (cc908a8) to head (eed8f9c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5888      +/-   ##
==========================================
- Coverage   79.03%   78.81%   -0.22%     
==========================================
  Files        1055     1067      +12     
  Lines      122233   123805    +1572     
  Branches     4401     4510     +109     
==========================================
+ Hits        96607    97581     +974     
- Misses      24061    24614     +553     
- Partials     1565     1610      +45     

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[njzjz]

Rebased onto master and made -j 2 actually work in eed8f9cfc.

The RTD build on this branch was failing at 655 s with:

NotImplementedError: merge_domaindata must be implemented in
<class 'dargs.sphinx.DargsDomain'> to be able to do parallel builds!

dargs.sphinx returns {"parallel_read_safe": True} from its setup, but DargsDomain inherits Domain.merge_domaindata, which raises. Sphinx then tears down the worker pool and the failure surfaces as a secondary KeyError in sphinx/util/parallel.py. doc/conf.py now installs a merge_domaindata on DargsDomain when it is still the base implementation; the domain's only state is a flat targetid -> (docname, objtype) map, so merging is a dict update restricted to the documents that worker read. Worth upstreaming to dargs, at which point the guard makes the shim a no-op.

Verified on a minimal two-page project using .. dargs:: directives: python -m sphinx -j 2 reproduces the exact NotImplementedError without the shim and reports build succeeded with it.

For context on why this matters beyond this PR: the docs/readthedocs.org:deepmd check is red on a large number of open PRs, and every one I sampled is the same 40-minute RTD wall-clock limit — builds 33625117, 33625500, 33626766 all ended at 2409-2411 s with the sphinx step still running. So this PR is the fix for that whole class of failures, not just a speed-up.