Make sure our links are updated to the new doc website

[MridulS]

We use https://docs.econ-ark.org/ instead of https://hark.readthedocs.io.

We should make sure we have updated all our docs and links to point towards the correct place.

A query to search for instances in the econ-ark organisation:

https://github.com/search?q=org%3Aecon-ark+hark.readthedocs.io&type=code