Website: slight layout tweaks

A few smaller tweaks for the navigation menu (on the left, not the top):

- Smaller font. The upgrade to the latest Geekdocs made this bigger, but I
  like it if it's a bit smaller than the main text.
- More visible indicator line for the active element. This was a dashed
  underline of just the text. It is now a dashed border-bottom that extends
  the entire width of the element. This visibly connects the main body of
  text to the active menu element.

And for the main text:

- Less padding for `h3` elements, as the whitespace became a bit too much.

web/project-website/static/custom.css

@@ -129,7 +129,6 @@ body {
 }
 
 h3 {
-  padding-top: 1rem;
   margin-bottom: 0.5rem;
 }
 
@@ -182,6 +181,21 @@ h3 {
   padding-left: 1rem;
 }
 
+.gdoc-nav nav section {
+  /* Make the font slightly smaller. */
+  font-size: 0.9rem;
+}
+
+.gdoc-nav__list li {
+  /* Squeeze the navigation items a little closer together. */
+  margin: 0.6rem 0;
+}
+.gdoc-nav__entry.is-active {
+  /* Make the current page in the documentation navigation tree more visible. */
+  text-decoration: none;
+  border-bottom: thin dashed var(--link-color-visited);
+}
+
 .flamenco-download-link {
   font-family: var(--font-family-mono);
 }