Mkdocs math support + more and better docstrings + interfaces fix

[loreloc]

Add Katex support to mkdocs

Minor fix in the interfaces: use Mapping instead of dict to hint immutability.

Added more docstrings, and fixed previous ones in the following modules:

• backend
  • torch
    • layers.{base,input,inner}
    • parameters.parameter
    • circuits
    • graph.modules
• symbolic
  • functional
  • circuit
  • parameters

TODO: add docstrings for the very long list of classes in backend.torch.parameters.nodes

Closes #303 .