Fix formatting of front page

[dlglin]

Links to Edfinity, preTeXt and LibreText need proper formatting.

[Alex-Jordan]

At first, we thought to make those links on the front page link directly to those three projects. The device that is used there for markup would not allow us to use direct links. So the links are too internal pages of the wiki, which then provide links to the three project sites.

Probably it is fine to have things as they are right now without external links on the front page.