Add more gn documentation

Adds new topics on labels, overall build flow, and ninja rule generation.
Enhances toolchain documentation. Removes duplicate content from language.md
(which I want to go away -- this should be moved into the builtin help).

Review-Url: https://codereview.chromium.org/2504133003
Cr-Original-Commit-Position: refs/heads/master@{#432766}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 189c461d55de98bab1f85abcac8e54203a1b9633
11 files changed
tree: 9d2849de1aca2256e3c78bb13a66eead450eaa2d
  1. tools/