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

docs/index.md

@@ -0,0 +1,12 @@
+# easy-tdx 文档
+
+```{toctree}
+:maxdepth: 2
+:caption: 目录
+
+readme
+api_reference
+field_mapping
+indicator-zhuoyao
+indicator-bias-signal
+```