docs: New pages

[leewujung]

• Quick start notebook: something like the previous EchoPro Python Workflow notebook
• Mathematical/theoretical description:
  • things like: converting NASC to abundance, abundance to biomass, kriging, CV analysis, etc
  • different sections mapped to algorithmic implementation
• Algorithmic implementation:
  • which variables is what symbol
  • how different variables are combined into another one and passed into a function to compute something
  • we will have different sections in code explained in different pages, ref echopype _toc
• Change title: Classes and data structures --> Core data structure
• Glossary
  • section: variable naming convention (_df, _arr, _gdf, tbl), if no "" it is a dictionary
  • section: physics-related quantities

[leewujung]

#193 addressed the first three bullet points.

[leewujung]

#195, #197, #201 together added all planned pages. I'll close this now and we can track detailed items in new issues.