Document the many ways of assigning a RasterIndex

[dcherian]

1. assign_index
2. RasterIndex.from_transform + assign_coords
3. set_xindex

Are there more?

I was thinking a new page titled Assigning would be a good place for this.

@benbovy can you take this one?

[benbovy]

set_xindex is not supported yet, but it could use a slightly refactored version of get_affine() added in #46.