docs(conf): add latin-ext subset for IBM Plex Sans and Mono

why: Accented characters (ñ, ř, ž, ö) in contributor names, docstrings,
and code comments fell back to system fonts. latin-ext covers U+0100-02FF
and extended Latin ranges used across European languages.

what:
- Change "subset": "latin" to "subsets": ["latin", "latin-ext"] for both
  IBM Plex Sans and IBM Plex Mono
- Total font files: 32 (2 fonts × 2 subsets × 4 weights × 2 styles)
- Zero performance cost for ASCII-only pages: unicode-range descriptors
  tell the browser to skip latin-ext downloads when no extended chars
  appear on the page

Author: tony

docs/conf.py

@@ -138,15 +138,15 @@
         "version": "5.2.8",
         "weights": [400, 500, 600, 700],
         "styles": ["normal", "italic"],
-        "subset": "latin",
+        "subsets": ["latin", "latin-ext"],
     },
     {
         "family": "IBM Plex Mono",
         "package": "@fontsource/ibm-plex-mono",
         "version": "5.2.7",
         "weights": [400, 500, 600, 700],
         "styles": ["normal", "italic"],
-        "subset": "latin",
+        "subsets": ["latin", "latin-ext"],
     },
 ]