Add tfp converter (or docs on conversion)

[OriolAbril]

ArviZ used to have a from_tfp converter, but it wasn't really maintaned so it eventually required pinning the tfp version and was eventually deleted in #1928. It would be nice to add it again and have it up to date.

Note: a documentation page along the lines of https://python.arviz.org/en/stable/getting_started/ConversionGuideEmcee.html but for tfp would be equally helpful (or even more if it were not to make sense to have a from_tfp converter anymore)