Documentation: how to create your own directives

[hamelsmu]

Need to create documentation on creating your own directives

cc: @CerebralMastication

[seeM]

IIUC this comes down to creating your own processor, since all directives (even unknown ones) are always parsed into the cell.directives_ dict. For example: https://github.com/muellerzr/nbdev-extensions/blob/fd2c0577f280e27853b63c622f07812d21cabd7f/nbdev_extensions/codenotes.py#L53

[hamelsmu]

@seeM do you want to tackle this since you added this feature ?