Add tour of LUCI markdown to src/docs

Bug: 785792
Change-Id: I6af0fb829b0adbddcf1d157da43a23e35a6acfe0
Reviewed-on: https://chromium-review.googlesource.com/831348
Reviewed-by: Andrii Shyshkalov <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Andrii Shyshkalov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#524910}
13 files changed