Add how-to guides related to multi-index set

• A guide on how to create a multi-index set in Minterpy has been added.
• A guide on how to add a set of exponents to a MultiIndexSet instance has been added.
• A guide on how to check for membership, superset, and subset of MultiIndexSet instances has been added.
• A guide on how to make a multi-index set complete
• All Jupyter-notebook-based documentation is now built using MyST-NB instead of NBSphinx (pandoc is no longer required).
• Remove unnecessary or redundant dependencies related to building the docs.
• The guides on polynomial integration have been updated with minor edits.

This MR should partially resolve Issue #113; I plan to resolve the issue one bit at a time potentially with some task delegation.