mdbook: Explain how to build a binary for beginner
mdbook: Explain how to build a binary for beginner

Add a section to explain how to build crosvm binary for beginners who
may not have used cargo.
Also, ran mdformat on the file.

BUG=none
TEST=check

Change-Id: I902157cd471652b829d73bb27fc52e04db64d6d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3252186
Auto-Submit: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Commit-Queue: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm