docs: restore ReadTheDocs support; modernize docs generation

We add a `.readthedocs.yaml` so RTD builds work again.

We add a dedicated requirements file for documentation.

Sphinx is updated to latest.

Author: indygreg

.github/workflows/sphinx.yml

@@ -19,9 +19,8 @@ jobs:
         env:
           PIP_CONSTRAINT: 'ci/constraints.txt'
         run: |
-          uv venv --python 3.12 venv
-          source venv/bin/activate
-          uv pip install -r ci/requirements.txt
+          uv venv --python 3.13 venv
+          uv pip install -r ci/requirements.docs.txt
           uv pip install --config-settings=--build-option=--no-c-backend .
-          uv pip install Sphinx==5.3.0
+          source venv/bin/activate
           make -C docs html

.readthedocs.yaml

@@ -0,0 +1,19 @@
+version: 2
+formats:
+  - htmlzip
+  - epub
+build:
+  os: ubuntu-24.04
+  tools:
+    python: "3.13"
+  jobs:
+    create_environment:
+      - python3 -m venv venv
+      - venv/bin/python3 -m pip install -c ci/constraints.txt -r ci/requirements.docs.txt
+      - venv/bin/python3 -m pip install --config-settings=--build-option=--no-c-backend .
+    build:
+      htmlzip:
+        - make -C docs html SPHINXBUILD=$(pwd)/venv/bin/sphinx-build BUILDDIR=$READTHEDOCS_OUTPUT
+      epub:
+        - make -C docs epub SPHINXBUILD=$(pwd)/venv/bin/sphinx-build
+        - cp docs/_build/epub/python-zstandard.epub  "${READTHEDOCS_OUTPUT}/"

ci/requirements.docs.in

@@ -0,0 +1,2 @@
+cffi
+Sphinx