Go over the documentation of the documentation system

[mmatera]

This PR is mainly about clarifying the code in the documentation system

• sort classes and functions in mathics.doc.common according to the order in doc-code-revision (and as in #981)
• adding docstrings
• changing some names of classes and functions to be more explicit about their role in the system
• adding Pytests for the documentation system.
• fix a typo (section ->section_all) in mathics.doc.latex_doc to allow load the GuideSections in the LaTeX documentation.
• fix an indentation error in the former gather_tests function previously reported in #981.

[rocky]

LGTM - let's merge and iterate.