doc: CSS: fix code fonts

Closes #10330.
doc: disable parser error highlighting + a few fixes

CodeMirror parser errors are related to #9873.
doc: add favicon to tutorial/manual

Since tutorial/manual files are stored on static.rust-lang.org, browsers
try to fetch the favicon from there while it should be retrieved from the
main domain.
doc: tidy up makefile rules
auto merge of #10421 : adridu59/rust/patch-www, r=alexcrichton

* Closes #10330
* Improves on #9873
* Adds favicon for tutorial files