Update readme and extend Pigweed documentation

Trims setup information out of the README.md, and pushes setup and
developer guides into their own pages.

Change-Id: I7fc80213e6a00c270fc9c664c8a7f54a19c05778
diff --git a/docs/images/pw_watch_build_demo.gif b/docs/images/pw_watch_build_demo.gif
new file mode 100644
index 0000000..877d849
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