Remove link to getting started, and whitespace
Open download page in new window.

Also, remove navigation highlighting logic, as there is no Download section
inside the documentation anyway.
Add known link to learning center.
Emit log messages if an uresolved link was found in the HTML output.

Very naive, just looking for ']['. Will clean up depending on how many
false positives we get.
Revert "Emit log messages if an uresolved link was found in the HTML output."

This reverts commit 1409262bcbc8bbd7bfa13032843fe689aac0dfd3. Writing
stuff to markdown files *after* the jekyll run is obviously not going to do
anything useful.

Instead, print to console.