mkdocs: auto-scroll to current section (ponylang#1155)

Irregardless of which section was viewed, the navbar would be displayed
_scrolled to the top_.  With this, it will scroll to the currently
viewed section.

Author: srenatus

.docs/extra.js

@@ -0,0 +1,2 @@
+// https://github.com/mkdocs/mkdocs/issues/803
+$(document).ready(() => $('.wy-nav-side').scrollTop($('li.toctree-l1.current').offset().top - $('.wy-nav-side').offset().top - 80));

.travis.yml

@@ -155,6 +155,7 @@ after_success:
       build/release/ponyc packages/stdlib --docs;
       cd stdlib-docs;
       sudo -H pip install mkdocs;
+      cp ../.docs/extra.js docs/;
       sed -i '' 's/site_name:\ stdlib/site_name:\ Pony Standard Library/' mkdocs.yml;
       mkdocs gh-deploy -v --clean --remote-name gh-token;
     fi;