chore: bump the pinned theme to v2.3.0

Takes the theme from v2.2.0 to v2.3.0, per the release notes in
QuantEcon/quantecon-theme.mystmd — git-history page headers (automatic,
from this repo's git log at build time), opt-in in-page live compute
via project.thebe (not enabled here), and CDN-free stylesheets
(KaTeX/jupyter-matplotlib self-hosted, Font Awesome dropped), so maths
renders correctly where jsdelivr/cdnjs are blocked.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/myst.yml

@@ -93,4 +93,4 @@ site:
   # Pinned theme release (see https://github.com/QuantEcon/quantecon-theme.mystmd/releases);
   # bump the vX.Y.Z in this URL to take a new theme version. The old bundled
   # QuantEcon/quantecon-theme repo is archived.
-  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.2.0/quantecon-theme.zip
+  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.3.0/quantecon-theme.zip