Separate documentation page for `TransferOperator`

[kahaaga]

Ref #190.

I guess this makes sense, since there is an entire layer beyond the probabilities estimation, which is the estimation of the invariant measure. More stuff will also be added once I get to address #55 .

[kahaaga]

In the 3.0 docs, TransferOperator is just another OutcomeSpace. It is therefore listed together with all the other outcome spaces under its own header, together with utility methods - just like for the other outcome spaces. This can be closed.