docs: add .readthedocs.yaml

This should hopefully fix docs build bustage.

Author: indygreg

.readthedocs.yaml

@@ -0,0 +1,22 @@
+version: 2
+formats:
+  - htmlzip
+  - pdf
+  - 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.txt
+      - venv/bin/python3 -m pip install Sphinx==5.3.0
+      - venv/bin/python3 -m pip install --config-settings=--build-option=--no-c-backend .
+    build:
+      htmlzip:
+        - make -C docs htmlzip SPHINXBUILD=$(pwd)/venv/bin/sphinx-build BUILDDIR=$READTHEDOCS_OUTPUT
+      pdf:
+        - make -C docs pdf SPHINXBUILD=$(pwd)/venv/bin/sphinx-build BUILDDIR=$READTHEDOCS_OUTPUT
+      epub:
+        - make -C docs epub SPHINXBUILD=$(pwd)/venv/bin/sphinx-build BUILDDIR=$READTHEDOCS_OUTPUT

docs/Makefile

@@ -6,7 +6,7 @@
 SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = .
-BUILDDIR      = _build
+BUILDDIR      ?= _build
 
 # Put it first so that "make" without argument is like "make help".
 help: