upload new gif
update data attribute on Betterlink script
redefine relative links from root

when creating a static page under a subdirectory (like
docs/advanced-setup.html), the existing relative links would attempt to
load CSS, images, and other files from within the same directory
increase default contrast of anchor links
switch domain to betterlink.io

- temporarily disable Google Analytics
- code.betterlink.co not migrated yet