[docs] glossary

add glossary to documentation with frequently used words, e.g. BrainModel, Benchmark, Metric, BaseModel, ...

I am a bit worried that things will become redundant because we already have parts of this written down e.g. in the [benchmarks module](https://brain-score.readthedocs.io/en/latest/modules/benchmarks.html).
Ideally we would find a way of maintaining these details in only one location and then taking parts from / linking to it in different places to avoid redundancy.