Support proper tutorial linking in docs on both repo and website

[Balandat]

This is an issue that came up in #754:

Currently, tutorial so the links in the .md doc files are such that the website points to the rendered tutorials (e.g. linking to https://botorch.org/docs/botorch_and_ax). With the .ipynb suffix the link on the website won't work anymore. Conversely, when following the link from a rendered .md on the repo, one will get a 404. There probably is a better solution that allows to do both (e.g. do some regexp parsing & editing of the .md files during the website building that removes ipynb suffixes).