Introduce a definitions page in the docs

To have consistent names with definitions (and links to further info) in the docs seems like it would be useful. I think this might take some time to agree on, so might be nice to brainstorm a bit here what can go in. I don't think this should block 1.0, we can always improve the docs by tagging a doc-version.

# Ideas definitions (please edit post and extend)
* [ ] `Node` (split geometric / algebraic)
* [x] `ReferenceShape` (#935)
* [ ] `cell`, `facet`, `face`, `edge`, `vertex` (and corresponding indices)
* [ ] `AbstractCell`
* [ ] `Interpolation`
* [ ] Degree of freedom
* Edit to add more...