[docs] intro to Brain-Score philosophy

The documentation should have more of a primer on what Brain-Score is and what its goals are.

A lot of this is in the [Neuron paper](https://www.cell.com/neuron/fulltext/S0896-6273(20)30605-X), maybe we can take parts from it and link to it for more details

Start by locating the repository's documentation entry point, then read the linked Neuron paper for the description of Brain-Score and its goals. Done means the documentation includes a clear introductory primer and links to the paper for further detail.

Written by the indexing model from the issue text.