Add the user agent content into the docs repo.

Original content: https://developer.chrome.com/multidevice/user-agent

Also, fixed some typos in opening_links.md.

[email protected]
BUG=

Review-Url: https://codereview.chromium.org/2778973002
Cr-Commit-Position: refs/heads/master@{#460051}
2 files changed