Move documentation from Shellfish

[TonioGela]

We need to import also all the documentation that @Hombre-x wrote for Catscript, being sure to change all the mentions in both code and documentation/comments from "shellfish" to "catscript".

This is softly blocked by davenverse/shellfish#86, that I'll take care to review and merge ASAP.

[Hombre-x]

Hi again! Ill do it as well

[TonioGela]

@Hombre-x please remember to do not add doc pages containing just a Coming Soon please.

[TonioGela]

It would be nice to have a couple of lines in the README too, please.