docs: publish to GitHub Pages instead of ReadTheDocs

[ondave]

Moves documentation hosting from ReadTheDocs to GitHub Pages.

Docs will be served at https://oceanum.github.io/xarray-video/. Pages is already enabled on this repo with GitHub Actions as the source, so the site publishes on the first push to main after this merges.

What changed

• .github/workflows/docs.yml (new) — builds the Sphinx docs on every push, PR and manual dispatch; deploys to Pages only from main. PRs get a build-only run, so a broken docs build is caught before merge.
• docs/requirements.txt — declares sphinx explicitly (it was only arriving transitively via pydata-sphinx-theme) and drops the xarray_video entry, which on CI would have installed the released package from PyPI instead of the checkout.
• .gitlab-ci.yml (deleted) — its pages job just copied a pre-built docs/_build/html. Dead now that the repo lives on GitHub.
• docs/_build/ — 86 committed build artifacts untracked, and the .gitignore rule (which was commented out to allow the GitLab job to consume them) re-enabled.
• README.md, README.rst, setup.py — documentation links repointed. In setup.py the link moves from a documentation= kwarg, which is not a valid setup() option and was silently discarded, into project_urls, where setuptools actually reads it. README.rst also picks up the oceanum-io → oceanum org rename.

Note on not installing the package

The docs job installs the runtime and docs requirements but not xarray-video itself. docs/conf.py already does sys.path.insert(0, os.path.abspath("..")), so autodoc imports the package from the source tree. Installing it is a trap: setup.py imports xarray_video to read __version__, so a PEP 517 isolated build fails on the missing numcodecs, and --no-build-isolation then fails on the missing setuptools in a Python 3.12 venv. Skipping the install sidesteps both.

Verification

Ran the workflow's exact steps locally in a clean Python 3.12 venv against a clean tree:

• pip install -r requirements.txt && pip install -r docs/requirements.txt — OK
• sphinx-build -b html docs docs/_build/html — exit 0, 22 HTML pages, 5 warnings (all pre-existing, see below)
• autodoc output confirmed to contain real API content (H264, VideoDataset, VideoArray, open_video class pages)
• no xarray-video.readthedocs reference remains in sources or built output
• setup.py still executes; metadata now emits Project-URL: Documentation, https://oceanum.github.io/xarray-video/

Pre-existing warnings, deliberately not fixed here

Out of scope for a hosting migration, but worth a follow-up:

1. language = None in docs/conf.py is invalid; Sphinx falls back to en.
2. html_static_path entry _static does not exist.
3. docs/xarray_video.codecs.rst autodocs xarray_video.codecs.mp4, a module that no longer exists.
4. docs/modules.rst is not in any toctree.

The build is not run with -W, so these do not fail CI.

Follow-ups outside this PR

• The repo homepage field still points at xarray-video.readthedocs.io; worth switching to the Pages URL once the first deploy is green.
• The ReadTheDocs project itself is configured in RTD's web UI (there is no .readthedocs.yaml in the repo), so it needs to be archived or deleted there. Until then RTD builds will fail, because docs/requirements.txt no longer installs the package.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Sv6cKAFfxedP6NjhEYLTdE