build: pin the theme to quantecon-theme.mystmd v2.5.0

Takes the icon-only search trigger, PT Serif headings, QuantEcon-blue links,
the Colab-only launcher and the first-paint fixes released today. Bumped
from v2.3.0, so v2.3.1 (static-build asset URLs) and v2.4.0 (content
typography, inline-code regression) come along too.

See QuantEcon/quantecon-theme.mystmd/releases/tag/v2.5.0.

Co-Authored-By: Claude Fable 5.1 <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/v2.3.0/quantecon-theme.zip
+  template: https://github.com/QuantEcon/quantecon-theme.mystmd/releases/download/v2.5.0/quantecon-theme.zip