No documentation for extending Fuzzbox

[mmrwoods]

Fuzzbox can be easily extended with custom commands, custom selectors, and extensions, but none of this is clearly documented.

For now, please see:

• https://github.com/vim-fuzzbox/fuzzbox.vim/blob/main/autoload/fuzzbox.vim for info on creating custom commands and selectors
• https://github.com/vim-fuzzbox/fuzzbox-foo.vim as an example of how to create an extension as a separate plugin.

Files directly inside autoload/fuzzbox provide a public API for extensions. You should not import from autoload/fuzzbox/internal.

Related to: #134