[lldb][Docs] Add simpler "automatic" cross-compile option to build docs (#65311)

The main way I cross build lldb is to point CMake at an existing host
build to get the native tablegen tools. This is what we had documented
before.

There is another option where you start from scratch and the host tools
are built for you. This patch documents that and explains which one to
choose.

Added another arm64 example which uses this. So the frst one is the
"automatic" build and the second is the traditional approach.

For ease of copy paste and understanding, I've kept the full command in
each section and noted the one difference between them.

Along the way I updated some of the preamble to explain the two
approaches and updated some language e.g. removing "just ...". Eveyone's
"just" is different, doubly so when cross-compiling.