docs(conf): add IBM Plex Mono weights 500/600/700

why: Only weight 400 was downloaded. Bold monospace text (badge labels at
font-weight 650, <strong> in code) used browser faux-bolding which looks
noticeably worse than the real typeface weights.

what:
- Match IBM Plex Sans weight range: [400, 500, 600, 700]

Author: tony

docs/conf.py

@@ -170,7 +170,7 @@
         "family": "IBM Plex Mono",
         "package": "@fontsource/ibm-plex-mono",
         "version": "5.2.7",
-        "weights": [400],
+        "weights": [400, 500, 600, 700],
         "styles": ["normal", "italic"],
         "subset": "latin",
     },