Improve X.A.Navigation2D documentation

[geekosaur]

Problem Description

It is not clear from a cursory reading of the documentation when to use lineNavigation vs. sideNavigation vs. centerNavigation; you have to dig into the documentation for each strategy, and they aren't conveniently linked from the summary.

Checklist

• [x] I've read CONTRIBUTING.md

• I tested my configuration: N/A

[geekosaur]

[21 16:41:42]  I am curious, is there any reason that lineNavigation is the default navigation in X.A.Navigation2D module?
[21 16:43:44]  Seems that with the default lineNavigation, movement between master and two slaves windows gets stuck
[21 16:45:52]  "Line navigation and Side navigation feel rather natural but may make it impossible to navigate to a given window from the current window, particularly in the floating layer. Center navigation feels less natural in certain situations but ensures that all windows can be reached without the need to involve the mouse."
[21 16:46:27]  there's also a paper presenting the details of the navigation strategies: http://www.cs.dal.ca/~nzeh/xmonad/Navigation2D.pdf
[21 16:46:47]  (I just verified it's not a dead link)
[21 16:46:53]  Whoa, what
[21 16:48:15]  alternateved: lineNavigation shouldn't get stuck in that situation (judging solely from its docs)
[21 16:48:38]  it might if you have gaps (we had that situation here a while ago)
[21 16:48:49]  I would need to test it once again, since I stopped using it.
[21 16:49:05]  But yeah, with gaps (created by Spacing) I had that issue.
[21 16:49:39]  the docs mention sideNavigation is being more intuitive for X.L.Spacing stuff
[21 16:50:00]  Ah, I see. So I guess that is my bias.
[21 16:50:37]  to be fair, one does have to look at the docs of the individual strategies
[21 16:51:25]  (and while I think that that module has excellent documentation, a quick list of all the strategies and their uses at the top level may be nice)
[21 16:51:50]  I guess explicit mention of the lineNavigation not working so well with Spacing would be good
[21 16:51:54]  there's already a list at the top, it just doesn't really say when to use them
[21 16:52:39]  and maybe instead of italics they should be links to the specific strategies