Merge pull request matplotlib#21607 from jklymak/doc-link-to-cheatsheetsgithub

DOC: link to cheatsheets site, not github repo

Author: timhoffm

doc/_templates/cheatsheet_sidebar.html

@@ -1,7 +1,7 @@
 
 <div class="sidebar-cheatsheets">
   <h3>Matplotlib cheatsheets</h3>
-  <a href="https://github.com/matplotlib/cheatsheets#cheatsheets">
+  <a href="https://matplotlib.org/cheatsheets/">
     <img src="_static/mpl_cheatsheet1.png"
          alt="Matplotlib cheatsheets"
          srcset="_static/mpl_cheatsheet1.png 1x, _static/mpl_cheatsheet1_2x.png 2x"/>