Restructure the documentation and add a tutorial

[lukaszcz]

• Closes Make the standard library documentation more easily accessible #1597
• Closes How-to Install Juvix #1624
• Closes Learn Juvix in X minutes | Tutorial  #1633

The tutorial uses syntax which has not been implemented yet: it depends on

• Typing for variables in let expressions #1637,
• Special syntax for case #1716,
• Pipes for lambda clauses #1639,
• Recursive lambdas #1638.

The tutorial also assumes the following issues are done:

• Evaluating strings in the REPL doesn't work #1720, and
• Make || and && lazy #1701.