Add examples of passing custom concept maps to poly

`poly` has a constructor that allows passing a custom concept map. This can be used like

```c++
if (condition) {
  dyno::poly<Concept> poly{x, concept_map_1};
} else {
  dyno::poly<Concept> poly{x, concept_map_2};
}
```

We should find a nice use case and document this.
