Feature the 'cabal' tool more prominently in the docs.

The docs/website sorely need to be reworked, but this will do for now.
Remove italic markers, they are parsed incorrectly by Pandoc.

See jgm/pandoc#976.
TOC update.
Docs for 'cabal repl'.
Add docs for sandboxes.
Merge pull request #1430 from 23Skidoo/docs-update

Describe new features in the documentation.