Add docs for user-guide

[cmdel]

Added procedures for starting, joining, sharing a meeting, and using on mobile.
Includes screenshots. Has some simple table of contents for user-guide to the Basic and Advanced usage and also a table of contents for the Basic usage to link to the processes.

[toby63]

Note: To avoid confusion: I am just a user and not representing the jitsi team.

First of all thanks for your effort, but I see some mistakes and room for improvement.

• Regarding the join guide (https://github.com/jitsi/handbook/pull/48/files#diff-7032bf7d68de8a666304975fc5439e6c):
  This is a good start, but:

  1. It should describe the scenario for every server, not just meet.jit.si.
     Also to avoid confusion, people should get the full URL by the server admin and then type in that URL, instead of typing in a server url and then the room name (this avoids potencial for man-in-the-middle attacks (though this might be an unlikely scenario)).
  2. This:
     Upon creating a [secure room name](../faq#_2-use-a-different-room-name-for-each-meeting--conference-you-have_)
     is missleading, the user wants to join a room and not create one, so we shouldn't confuse users with that.
     We should describe joining your own room in the start-guide imo.
  3. No offence but this is very basic.
     Especially the part about granting access to microphone and camera needs a bit more formulation.
     Also other parts might be important, like setting a username etc., though we can debate whether that should be on a different page.

• Regarding the start guide (https://github.com/jitsi/handbook/pull/48/files#diff-ec736629e98d7e1623d7acdb7c8bb8ab):
  This is not sufficient.

  1. You cannot give people a static url and tell them to use it.
     They need to use their own string every time.
  2. This should describe a scenario for every server, not just meet.jit.si.
  3. Also we need to describe more details:
     • setting a password
     • other settings
     • how to invite users/send users the url
  4. The screenshot should be from a computer, because the scenario for mobile can be described in the mobile guide.

• Regarding the mobile guide (https://github.com/jitsi/handbook/pull/48/files#diff-c2e2cd14aa1f46ead17d5814711d6c6e):

  1. I think WiP - Jitsi on mobile article #42 is and will be a bit more advanced, so we might pick that or combine both.
     But we might discuss whether we should seperate iOS and Android (like you did).
  2. Same points as above:
     • You cannot give people a static url and tell them to use it.
       They need to use their own string every time.
     • This should describe a scenario for every server, not just meet.jit.si.
     • Also it should clearly seperate between joining and starting a room.

• Regarding the screenshots:
  They might be improved as well.

  • join-jitsi-web.png for example might need some editing, we could include a sign that points to the right positions.
  • jitsi-meet-ios-app.jpg you need to delete the log for past connections properly, I suggest to simply paint it all over with the background color of the app.
  • jitsi-meet-ios-install.jpg is not so perfect, we need a screenshot with the "install" button visible.

Sry if this seems very negative, but this should be done properly to help users 🙂.