Add a link to Sentinel View and document how to get the token

The link to https://view.sentinel.turris.cz/ is not mentioned in our documentation.

Also, people ask how to get the token for https://view.sentinel.turris.cz/devices/, which is:

uci get sentinel.main.device_token

It is mentioned in Filip's blog post. The reForis based way should be added later when released to HBS.