docs: add Sphinx + Read the Docs infrastructure

- Add .readthedocs.yaml build config (Ubuntu 22.04, Python 3.11)
- Add docs/conf.py with myst-parser for Markdown support
- Add docs/index.md toctree including README and existing docs
- Add docs/readme.md to include root README via myst directive
- Add docs/requirements.txt for Sphinx build dependencies
- Add docs/_build/ to .gitignore

.readthedocs.yaml

@@ -0,0 +1,18 @@
+# .readthedocs.yaml
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+sphinx:
+  configuration: docs/conf.py
+
+formats:
+  - pdf
+  - epub
+
+python:
+  install:
+    - requirements: docs/requirements.txt