Update docs

[vasilismsr]

Documentation is outdated for most of our estimators due to the several updates we have done on the code. We need to an overall update to our docs to sync with the current state.

[kbattocchi]

See also #80

[vasilismsr]

#160 #161 #159 partially address this.

The ortho_forest and meta learners user guide still need to be enhanced to follow the same pattern as the new dml user guide.

Also the API section needs more polishing.

The inference user guide needs some mentioning of the other methods of inference used in the library.

[kbattocchi]

Closing as I believe these changes were addressed long ago.