Add documentation for builtin commands (for "end-users", not library using developers)

[vorburger]

IMHO in a perfect world it could be neat if somewhere on the https://jline.org documentation website there was documentation for builtin commands.

Ideally I would document them for "end-users", not library using developers, so that documentation of projects using JLine can point their users to this page; saying something like: "You can customize key bindings using the keymap command, which we inherit from the amazing JLine library." and linking to that page.

I would focus on these, which to me seem "lower level" (as in, any application using JLine could benefit from these):

• history
• widget
• keymap
• setopt
• unsetopt

and perhaps also these as they are generic and cute:

• ttop
• colors

but I would perhaps not docment the following as prominently, or maybe on a separate pages, because these (to me) seem "higher level", and possibly not of interest to all application using JLine as a library:

• nano
• highlighter
• less
• setvar

[gnodet]

The builtins module documentation at https://jline.org/modules/builtins has been updated with a list of all available commands and their options. Each command also supports --help for inline help.

If you're looking for more detailed usage documentation for specific commands (like less navigation keys or nano shortcuts), let us know which commands need more coverage.