platform_testing: Fix navigation, misc links and image

A bunch of cleanup as this is being rendered by gitlites and not
g3 docs.

* Navigation needs to use navbar.md not sitemap.md
* Links have to be on the same line.
* The image was not properly uploaded.

Bug: None
Test: None
Change-Id: I82e27bad267f3720f566e197d2cf826b1a3cb703