Update the best practices documentation

This change replaces an internal link with an external one from the
Google style guide. It also repairs a broken markdown link.

BUG=799576
TEST=Launch md_browser with http://localhost:8080/docs/documentation_best_practices.md

Change-Id: I816aefde95fbbf0d3a365504993e251e5961cc66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570573
Reviewed-by: Dirk Pranke <[email protected]>
Commit-Queue: Dirk Pranke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#651569}
1 file changed