01_Application_Framework: Moved D-bus actvation to its own page

The 02_Application_Startup pages actually contains quite a lot of D-Bus
specific bits so move it to its own page, rather than adding items about
the gRPC one. The gRPC part are inlined into the current page though.

Bug-AGL: SPEC-4700
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I4808cc1773f071f2455e24b5417a8b30337cbb8a
(cherry picked from commit 2e981330615aff75b16c69062a6302c858660590)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/28540
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>