Revised Getting Started to be easier to follow and up to date

- Removed information on @ bootstrap
- Attempted to call out steps more clearly
- Added more pointers to additional information
- Extracted the "Explore Fuchsia" section so it wouldn't be intermixed
with qemu instructions
- Fixed verb parallelism errors in headings

TO-152 #done

Change-Id: Ie27acfad84fc59ef4cfa3d8004c5b948d3a93cbd