Add documentation on builtin types and functions

[developedby]

Is your feature request related to a problem? Please describe.
Currently builtins are each explained in documentation (lists, strings, etc), but there is nowhere that actually lists all the builtins and explains what they are for.
Everything is sort of hidden in builtins.bend.

Describe the solution you'd like
A document explaining each builtin.

Describe alternatives you've considered
Somehow making builtins.bend accessible for users, like automatically generating documentation from programs ala doxygen, docs.rs, etc.

Additional context
Users have complained about this.
There's actually hidden features in the builtins (maps).

[Sipher]

Wrote a .md file to help out, not sure where to add in the Readme,will send a PR if enjoyed and if necessary I can edit the file.

Here's a gist for a temp file at least:

https://gist.github.com/Sipher/088d26319f70745c2c5144f9ff3667e7

If you don't like it I can just redo or undo