Pin pydata-sphinx-theme to 0.14 (#390)

- 0.15 introduces spurious "Section Navigation" sidebars in pages which
have no sections (e.g. the Install guide) and 0.16 breaks Fontawesome
icons.
- In addition, 0.14 is the version used on Netlify previews, so we'll be
sure that PR previews correspond to the deployed version.

requirements.txt

@@ -1,4 +1,4 @@
 pre-commit>=2.10.0,<4  # For pre-commit hooks; lowercap to config syntax
-pydata-sphinx-theme>=0.14.1,<0.17  # Theme; lowercap to add version warning
+pydata-sphinx-theme==0.14.*  # Theme; lowercap to add version warning
 sphinx>=5,<9  # Docs generator; cap to range confirmed supported by deps
 sphinx-design>=0.5.0,<0.7  # For dropdowns and panels; lowercap to sphinx>=5,<8