docs: use a lean conda env for Read the Docs builds

[steven-murray]

Every Read the Docs build since mid-2024, for PRs and for latest/main alike, has been killed at RTD's 15-minute limit while running mamba env create --file ci/test-env.yml. The last successful build was 24721410 on 2024-06-17. Recent examples: 34949820 (#91) and 34975639 (latest at 7b35331).

ci/test-env.yml is too expensive to solve within that limit. It uses three channels (including the old conda-forge/label/cf201901), pulls in all the samplers and test tooling, and leaves python and 21cmFAST unpinned. A dry-run solve takes about 11 minutes locally with micromamba 2.9, and longer with the mamba 1.x in RTD's mambaforge-23.11 image.

Changes

• New docs/environment.yaml, conda-forge only: python=3.11, pip, 21cmfast>=3.3,<4 (needs conda for FFTW/GSL), and pytorch-cpu (stops pip pulling CUDA torch for py21cmemu). RTD's existing pip install .[docs] supplies everything else.
• .readthedocs.yml now points conda.environment at the new file, with a comment explaining why.

ci/test-env.yml is unchanged.

Verified locally

• The new env solves in about 5s.
• After pip install .[docs], py21cmfast, py21cmmc and torch all import.
• sphinx-build -b html docs succeeds with 21 warnings. These come from existing docs content: bad link targets in AUTHORS.rst, and a duplicate "Notes" section that stops Core21cmEMU from being autodoc'd.

Follow-up: INSTALLATION.rst still points users at the slow ci/test-env.yml.

🤖 Generated with Claude Code

Summary by Sourcery

Configure Read the Docs to build documentation with a lean, conda-forge-only environment that avoids timeout-prone dependency solving.

Bug Fixes:

• Restore Read the Docs builds by replacing the slow full test environment with a lightweight documentation-specific environment that solves within the build time limit.

Enhancements:

• Pin the documentation environment to Python 3.11 and provide the conda dependencies required for 21cmFAST and CPU-only PyTorch.
• Keep the full CI test environment unchanged while allowing documentation dependencies to be installed separately through the existing docs extra.

Documentation:

• Configure Read the Docs to use the new lean documentation environment.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Introduces a lean, conda-forge-only documentation environment with pinned Python and constrained 21cmFAST, then configures Read the Docs to use it instead of the expensive full test environment. The existing pip docs installation remains responsible for the rest of the documentation toolchain, improving build solve time without changing CI test dependencies.

File-Level Changes

Change	Details	Files
Replace the slow CI environment with a minimal, pinned conda environment for documentation builds.	Add a conda-forge-only environment with Python 3.11, pip, 21cmFAST constrained to the supported 3.x range, and CPU-only PyTorch. Keep compiled 21cmFAST dependencies in conda while relying on the existing pip docs extra for remaining documentation dependencies.	docs/environment.yaml
Configure Read the Docs to use the lean documentation environment.	Point the RTD conda configuration at the new documentation environment. Document that the full test environment exceeds RTD’s 15-minute dependency-solving limit.	.readthedocs.yml

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.99%. Comparing base (e28fc83) to head (105a013).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #92   +/-   ##
=======================================
  Coverage   73.99%   73.99%           
=======================================
  Files           8        8           
  Lines        2123     2123           
=======================================
  Hits         1571     1571           
  Misses        552      552           

Flag	Coverage Δ
unittests	73.99% <ø> (ø)

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