Document the printing features in the user documentation

[SoongNoonien]

Is your feature request related to a problem? Please describe.
From a users perspective it is currently not so clear which features regarding the output of data structures are provided by OSCAR. Specifically I think about the unicode stuff. Right now a new user will probably never discover the unicode printing because the only place one can read about it in the docs is inside the developer section.

Describe the solution you'd like
I'd suggest a new page "Printing" under the section "General" which illustrates the unicode printing and possibly even the latex/html printing.

[fingolfin]

So this is about allow_unicode and related functions in AbstractAlgebra, I think.

[fingolfin]

Also is_unicode_allowed and with_unicode.

and it seems one can use https://github.com/JuliaPackaging/Preferences.jl to also store the preferences in a file -- I think @lgoettgens may have added this and may be able to tell us more about how to use it (or perhaps @emikelsons can figure it out himself)

[lgoettgens]

and it seems one can use JuliaPackaging/Preferences.jl to also store the preferences in a file -- I think @lgoettgens may have added this and may be able to tell us more about how to use it (or perhaps @emikelsons can figure it out himself)

allow_unicode(::Bool) uses Preferences.jl by default (unless setting temporary=true) to persist the setting across sessions. This is explained in its docstring.
@emikelsons if you have any questions concerning docstrings or things maybe missing from there, just ping me.