Clarify quickstart documentation.
Clarify quickstart documentation.

When I joined the team, I had a hard time interpreting some of the
Quickstart steps. If you run them as presented, you get seven different
output directories. I've added some text to explain that you don't
actually need this many, and I've also fixed an anchor link.

I made these edits in http://cider/ for its excellent previewing
capabilities, and Cider automatically reformatted and rewrapped the
Markdown to match google3 standards. (e.g. using # and ## to denote
sections, wrapping code blocks in ```.) This doesn't affect the
generated output.

Change-Id: I118a6ed5f6848c845c102ac1670a5d16e79c71fe
No-Try: true
Docs-Preview: https://skia.org/?cl=306717
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306717
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

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