Fix links to commit references
Replace arrows in mathmode to unicode arrows

Avoid using the math extension. The arrows looked fine on my local build but
on the online documentation they weren't rendering properly.
Remove outdate installation info

I also noticed we currently have 3 different pages for installing Freeseer:
1. http://freeseer.github.io/docs/quick-start.html
2. http://freeseer.github.io/docs/userguide/install.html
3. http://freeseer.github.io/docs/contribute/install.html

We should simplify that and preferably have one page that we cross-reference.

Relevant commits:
3ca9f42360be39b2f67531b5454b93ea17b728e4
75c7c2f25c25ce2c04fc61ea6323a508819dcbe2