Use "approximate" instead of "experimental" for Pathfinder and ADVI descriptions #3207

Summary:

Both Pathfinder and ADVI algorithms are approximate algorithms. Craft console messages accordingly and revise the documentation to explain what this means.

Description:

The ADVI implementation lives in a folder called "experimental" which reflects what was known about the algorithm at the point that it was added to Stan. ADVI is no longer "experimental" in that it has been field-tested for the past 5 years.

Pathfinder has only been tested against PosteriorDB so far; it will be added to the next Stan release.

Current Output:

ADVI spits out a block warning on the console, which quickly scrolls away. Revise.

Current Version:

v2.32.2