Our documentation needs nightly features `doc_cfg`, `doc_auto_cfg` to
build. Instead of using the nightly channel, which is not available on
readthedocs.org, we opt in to nightly features by setting the
environment variable `RUSTC_BOOTSTRAP` to `1`. This gives on unstable
features in a stable channel.
Our dependency `proc-macro2` tests if opting-in to unstable feature is
possible, and if so, it expects that it runs on a quite recent `nightly`
release. The newest rust version that is available out-of-the-box on
readthedocs is rust 1.82, which is not *that* recent anymore, being
released on 2024-08-30.
This PR makes use of the readly configured tool `asdf` to select a more
recent rust version semi-manually.