document sticky footer solution, per stefanv's out of band PR review

Author: brianhawthorne

site.css

@@ -7,11 +7,22 @@ body {
 }
 
 .article.content {
+  /* Override 100vh from myst-theme:styles/typography.css so content div
+   * doesn't push the footer offscreen.
+   */
   min-height: 0vh;
   flex: 1 0 auto;
 }
 
 .footer {
+  /* Make footer "sticky" to page bottom (also the above flex rules), per
+   * the flexbox strategy described here:
+   * 	https://css-tricks.com/couple-takes-sticky-footer/#aa-there-is-flexbox
+   * and here:
+   *    https://philipwalton.github.io/solved-by-flexbox/demos/sticky-footer/
+   * This solution does not require hardcoding a fixed footer height in the
+   * style rules.
+   */
   flex-shrink: 0;
   background: #013243;
   color: white;