Allow access to latest documentation version through stable URL

[corykinney]

As it is now, if I were wanting to link to the ct.Reactor documentation I would link to https://cantera.org/documentation/docs-2.6/sphinx/html/cython/zerodim.html#reactor. When Cantera 3.0 is released, the link will no longer point at the latest documentation. It would be nice if the latest version of the documentation were always accessible at a stable URL; for instance, with NumPy the latest version of documentation for np.nan would always be at https://numpy.org/doc/stable/reference/constants.html#numpy.NAN.

I am proposing that the URL https://cantera.org/documentation/stable/... would currently host the 2.6 documentation, until the point at which 3.0 is released. At that point, the same URL would then point at the 3.0 documentation, and the 2.6 documentation would be moved to https://cantera.org/documentation/2.6/.... It would be especially helpful if there were a version selector on the documentation page in this case.

On a related note, it would also be nice if the URLs were shorter. Currently the python documentation is prefixed by sphinx/html/cython which is understandable, as it is informative of the underlying implementation, but I think simply using python as the prefix could be much more user friendly. Similarly, substituting docs for documentation. For example: https://cantera.org/docs/stable/python/zerodim.html#reactor.

I'd be curious to hear others thoughts on this. I'm not sure if there are any limitations on how this could break existing links.

[ischoegl]

Linking some old discussion from Cantera/enhancements#150 ...