Doc: Update the RTD documentation

[johnmhoran]

We want to bring the RTD up-to-date.

[mjherzog]

This Issue is to cover some important immediate work to upgrade the existing ScanCode Workbench documentation at https://scancode-workbench.readthedocs.io/en/develop/. The current goal is to get a new v3 baseline for documentation based on WB v3.1.1.

The specific near-term content requirements are to:

• Restructure the TOC/Index to emulate what we have for ScanCode Toolkit with logical adaptations like UI Reference for WB instead of CLI Reference for TK
• Fill in major missing topics like Getting Started, How-To Guides and Tutorials with content from WB v3
• Replace the old animated GIFs with single screenshots like we have already for Conclusions (courtesy of Steven) - see Doc: Update UI Reference Section #441
• Migrate the relevant content from the Wiki in GH to RTD and remove from GH Wiki when done
• Replace the current functional doc in the top-level README.md with reference to RTD site.

We also need to use this cycle to document how to maintain RTD websites in general for AboutCode projects including how to make pull requests, how to run the "build" etc. This documentation probably belongs at https://aboutcode.readthedocs.io/en/latest/index.html# since it should be basically the same across our projects. It may make sense to take a different "mass-update" approach for the first round of work on upgrading the WB doc.