Add a "Key Features of scores" page to the documentation

[Steph-Chong]

Submitted PR #567

The PR adds a "Key Features of scores" page to the documentation.

It is very close (but not identical) to the key benefits section of the JOSS paper draft.

My logic for suggesting this for inclusion in the docs, is to make it easier for potential new users to assess if scores is likely to be of interest to them. I realise most of the information is already in the README, but due to the need and desire to include other information on the README page, some of the key feature information may not jump out as much as it once did. So I think this page could be potentially useful.