Fix Read the Docs builds broken by Ubuntu 26.04 image

[gagelarsen]

Description

This merge request fixes the Read the Docs build, which has been failing for every PR since late June (e.g. #1277, #1282, #1284, #1285, #1287, #1288). Read the Docs flipped its ubuntu-lts-latest alias to Ubuntu 26.04, and that image only supports the miniforge3-* conda toolchain. The deprecated mambaforge-4.10 pinned in .readthedocs.yaml fails to install on it:

asdf install python mambaforge-4.10.3-10
...
EnvironmentLocationNotFound: Not a conda environment: .../conda/<version>
BUILD FAILED (Ubuntu 26.04 using python-build 2.8.0)

conda-forge retired Mambaforge in favor of Miniforge3 (functionally identical since Miniforge3-23.3.1-0), and Read the Docs deprecated all mambaforge-* values. A Read the Docs maintainer confirmed miniforge3-* is the supported replacement on the new image (readthedocs/readthedocs.org#13149); other projects hit by the same break applied the same one-line fix (e.g. MDAnalysis/mdanalysis#5410).

Changes Made to Code

• .readthedocs.yaml: build.tools.python changed from deprecated mambaforge-4.10 to miniforge3-latest.

Related PRs, Issues, and Discussions

• Why this error on build? readthedocs/readthedocs.org#13149
• Build: retire Mambaforge (deprecated) and replace it with Miniforge3 readthedocs/readthedocs.org#11690 (Mambaforge retirement)
• Unblocks the docs/readthedocs.org check (and therefore Merge Gatekeeper) on all open PRs, including Fix database access warning during app initialization with Django 5 #1288.

Additional Notes

• The docs/readthedocs.org:tethys-platform check on this PR is the real verification — it builds with this config.

Quality Checks

• At least one new test has been written for new code (N/A — CI config only)
• New code has 100% test coverage (N/A)
• Code has been formatted with Black (N/A)
• Code has been linted with flake8 (N/A)
• Docstrings for new methods have been added (N/A)
• The documentation has been updated appropriately (N/A — config change fixes the docs build itself)

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