Update AboutCode "TOC" information in README and RTD

[mjherzog]

It is a good time to update this repo's README and our RTD "home page" to provide a consistent explanation of AboutCode and our projects. The README is more current than the RTD home page, but the information in both places is somewhat dated and the information is not consistent between them.

This issue is a proposal for a short-term fix to:

• Update the "What is AboutCode" and "Projects" information in the GH README.
• Replicate that information to the AboutCode home page on RTD with as much automation as possible so that we keep these in sync. There should be some Markdown to RST conversion tools to facilitate this.

It probably does not make sense to list every project but we should have a consistent list of the top projects and the supporting projects that should be listed in the Project index/TOC.

[mjherzog]

@adaaaam Dennis is working on the project list, but we probably need your help for: What is AboutCode? to update it to your latest positioning. The change version is in the new-readme branch.