Update basic tutorial

Nobody has claimed this yet.

The basic tutorial has fallen behind the Lisp-Stat project and needs to be updated. In addition to the need for a def/defdf explanation pointed out by @rieser, there are several sections in the documentation that are commented out (you only see this in the documentation source repo because Hugo removes comments from deployed documentation).

Essentially the basic tutorial is the XLisp-Stat tutorial, updated for (Common) Lisp-Stat. At the time of conversion, Lisp-Stat lacked much of the functionality mentioned in the tutorial, so it was commented out. Lisp-Stat has now closed that gap, but the basic tutorial has not kept up.

This would be a good first issue for someone that wants to learn statistics or common lisp.

Originally reported issue:
It would be good to introduce `defdf` somewhere in the Data Frame tutorial. While reading that section, I tried experimenting with my own .csv files, for instance, with `(read-csv my-csv-file-stream) \\ (def results *)`, using what has been introduced so far, and it does give me back a DATA-FRAME object, and I can do a lot of manipulations on it. However, if I try to access `results:my-key`, I get an error, which is extremely confusing. On the other hand, if I use `defdf` instead of `def`, as indicated in the Manual (but not the Tutorial), then `results:my-key` works as expected.

An alternative, of course, would be to extend `def` to handle data frames properly.

Review the basic tutorial in the documentation source repository, especially the Data Frame tutorial and the sections commented out before Hugo deployment. Compare the tutorial with the Lisp-Stat manual and current functionality, including the defdf explanation. Done means the tutorial reflects available functionality and no longer omits relevant sections.

Written by the indexing model from the issue text.