Add a "Using LLDB" section to the welcome page of the website

This is an attempt to feature the user-facing resources more
prominently on the LLDB website by calling out the tutorial and the
GDB command map wight on the start page.

I also moved the "Why a new debugger" section to the "Goals"
subpage. Given that LLDB's first release is almost a decade in the
past now, the title is a bit of an anachronism.

Lastly, I moved the Architecture sub-page from "use" to "resources",
since end-users do not care about the source code layout.

Differential Revision: https://reviews.llvm.org/D70449