Update docs mentioning `xtb-python`

[Andrew-S-Rosen]

The latest docs say to go to xtb-python if you want a Python interface, but the README of xtb-python points out that it is no longer supported and tblite should be used instead. Therefore, I recommend updating the xtb docs to instead point to tblite directly.

[awvwgk]

There is already a warning banner for the deprecation of xtb-python and a link for tblite, but it would be better to directly point to tblite