Support level 4 in the left navigation

(cherry picked from commit 8518be8c2061f257b7224d571fff3fe0bfe8fe9a)
Experimental - remove base_url

(cherry picked from commit 58b32957df8239330e2cfa4ad18522f272930e52)

Conflicts:
	_config.yml
Use a regular expression to try to detect unresolved links.

The naive grep'ing for ][ produces too many false positives from
code blocks using arrays.

(cherry picked from commit 9afaf43ff0972dad18caae3717963ba738c366e0)