Improve documentation

Nobody has claimed this yet.

This issue is to track two potential improvements in documentation:

- API reference
- System manual

**API Reference**
Lisp-Stat uses [declt](https://github.com/didierverna/declt) to generate reference documentation from doc strings and output HTML, PDF, epub, etc. For example see the [reference for data-frame](https://lisp-stat.github.io/data-frame/). This doesn't work too well for `statistics` because either:
1. There is no doc string
2. The doc string is in a comment preceding the function.

**System Manual**
It seems that the majority of new users are not from the Common Lisp community and often they're not well versed in statistics either. If we look at other statistical computing systems, like Python, Julia or R, they have robust documentation, with plenty of examples. See the [data frame system manual](https://lisp-stat.dev/docs/manuals/data-frame/) for an example.

Start by reviewing the existing data-frame reference and system manual, then inspect how declt handles statistics doc strings and comments. Clarify whether the work covers the API reference, the system manual, or both; done should be an agreed scope with the relevant documentation improvements published.

Written by the indexing model from the issue text.