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Expand navigation markdown documentation.

This CL revises navigation.md and splits out navigation_concepts.md
for describing several concepts that are important for
understanding the navigation logic.

BUG=1015882

Change-Id: I0397d2906aec9c24a658d96cf67123dc6bd7c3af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225745
Commit-Queue: Charlie Reis <creis@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782858}