We used to have an index and a page for the general info pointing to the
firmware description back when this documentation was disjoint from TRS.

It makes more sense now to present the firmware summary when clicking the
"trusted substrate" section directly instead of having to click
"general" -> "trusted substrate".  So let's get rid of the former and
simplify the layout a bit

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>