Docs: add CSS to improve left-hand nav nesting

.npmignore

@@ -8,6 +8,7 @@ test
 .travis.yml
 appveyor.yml
 mkdocs.yml
+docs/css/
 vendor
 .prebuildrc
 .nyc_output

docs/css/extra.css

@@ -0,0 +1,5 @@
+/* Nest document subheadings in navigation */
+ul.subnav ul:not(.subnav) {
+  padding-left: 2em;
+  font-size: 80%;
+}

mkdocs.yml

@@ -5,6 +5,8 @@ site_description: High performance Node.js image processing, the fastest module
 copyright: <a href="https://pixelplumbing.com/">pixelplumbing.com</a>
 google_analytics: ['UA-13034748-12', 'sharp.pixelplumbing.com']
 theme: readthedocs
+extra_css:
+  - css/extra.css
 markdown_extensions:
   - toc:
       permalink: True