Bump quantecon-book-theme from 0.20.0 to 0.21.0

Brings this edition in line with the English source and the fa and fr
editions, which all pin 0.21.0.

The range carries three patch fixes (citation emphasis colour in 0.20.1
and 0.20.2, h5 heading size in 0.20.3) and one layout change in 0.21.0:
at 1024-1499px viewports the content column now gets a left margin
instead of sliding under the sidebar. The release notes flag that
downstream sites will see a small shift there and should re-verify
visually.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

environment.yml

@@ -7,7 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book>=1.0.4post1,<2.0
-    - quantecon-book-theme==0.20.0
+    - quantecon-book-theme==0.21.0
     - sphinx-tojupyter==0.6.0
     - sphinxext-rediraffe==0.3.0
     - sphinx-exercise==1.2.1