Add a page that gives an overview of each repository component

See https://trello.com/c/R7o2B8RV/99-add-a-page-on-which-component-is-responsible-for-what.   Almost certainly needs more work, but some feedback on what is missing or doesn't make sense would be helpful.
perhaps some markup for these commands too, like,
```rst
:command:`showinf`
```