Improve manual

We need more content and rich media. The document structure can be improved too.

Proposed structure

- Migrate to https://docs.briarproject.org/ ??
- Content:
     About Briar:
         What is Briar
         How Briar works
     Installation:
         Google Play
         F-droid
         Apk
     Running Briar for the first time:
         Creating an account
     Contacts:
         Add Contacts:
             Add contact at a distance
             Add contact nearby
         Deleting contacts
         Introducing contacts
     Messaging:
     Private Groups:
         Create a group
         Invite contacts
     Forums
     Blogs:
         Write a blog post
         Reblog
         RSS Feeds
     Settings:
         Language & Region
         Theme
         Connections:
             Via the internet
             Via Wi-Fi
               Via Bluetooth
               Connection method for Tor network
               Use mobile data
               Connect to the internet only when charging
         Security:
             App lock
             Change password
         Panic button
         Notifications
         Feedback
       Updating:
          Google Play
          F-droid
          Apk
       Uninstalling
       Known issues/limitations