Fix docs build: downgrade jupyter-book to 0.15 for Sphinx compatibility

- Downgrade jupyter-book from 2.0 to 0.15.1 for Sphinx backend
- Jupyter Book 2.0 uses incompatible mystmd build system
- Add dftio/_version.py to .gitignore (auto-generated by setuptools-scm)
- Tested locally - build successful with all HTML pages generated

Author: QG-phy

.gitignore

@@ -104,6 +104,9 @@ ipython_config.py
 # uv
 .python-version
 
+# Version file auto-generated by setuptools-scm
+dftio/_version.py
+
 # pdm
 #   Similar to Pipfile.lock, it is generally recommended to include pdm.lock in version control.
 #pdm.lock

dftio/_version.py

@@ -31,7 +31,7 @@ dev = [
     "pytest>=7.2.0",
     "pytest-order==1.2.0",
     "pytest-cov",
-    "jupyter-book",
+    "jupyter-book>=0.15,<1.0",  # Use 0.x for Sphinx-based builds
     "sphinx-autodoc-typehints",
 ]