Port documentation from authoring.datacamp.com

[filipsch]

Currently, the documentation for sheets is located at authoring.datacamp.com, here to be more specific. To make things consistent with other SCT packages out there, we should pull these articles into in-package docs, built by readthedocs and available here.

A lot of the naming has changed to make it more consistent with the other what packages out there:

• check_value() --> has_equal_value()
• check_formula() --> has_equal_formula()
• check_regex() --> has_code()
• check_function() --> check_function()
• check_reference() --> no longer supported
• check_operator() -> check_operator()
• discontinue check_reference()
• check_references() + check_absolute_references() --> has_equal_reference(absolute=False/True)