[DEV] Update TOC in sidebar

[tallamjr]

Update TOC in sidebar to include new ideas laid out in README.md such as:

##### ASTROINFORMATICS.XYZ

* Overview
    * Introduction
* Data Science
    * Mathematical Foundations
    * Data Structures
    * Algorithms
* Data Engineering
    * Data Formats
    * Database Internals
* Research Engineering
    * Tools of the Trade
        - Version Control
        - Editors
        - TMUX
    * Best Practises
        - Git logs
        - Branching models
    * Testing