Make the 'learn' section on the index page consistent with the nav bar
tab. Prompt 'views' doc up a bit, and use an image(credit to @ellyjones)
instead of ASCII diagram for better understanding.

Bug: none
Change-Id: Iac2c59e3c28a0f5021bc398ffad80929435a797c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417072Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808583}