* docs: add a spack environment for building the docs
* docs: remove tutorial and link to spack-tutorial.readthedocs.io
The tutorial now has its own standalone website, versioned by instances
of the tutorial. Link to that instead of versioning it directly with Spack.
- Generating the HTML from for >2300 packages from RST in Sphinx seems to
take forever.
- Add an option to `spack list` to generate straight HTML instead.
- This reduces the doc build time to about a minute (from 5 minutes on a mac laptop).
