Improve documentation

[snunez1]

This issue is to track two potential improvements in documentation:

• API reference
• System manual

API Reference
Lisp-Stat uses declt to generate reference documentation from doc strings and output HTML, PDF, epub, etc. For example see the reference for 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 for an example.

[jgarte]

hi,

The home page also currently points to the main lisp-stat page instead of dedicated documentation:

https://lisp-stat.dev/docs/

[Symbolics]

Which link is pointing to the main page? The github main page?

[jgarte]

Search for "Explore the docs" here:

https://raw.githubusercontent.com/Lisp-Stat/statistics/master/README.md

[Symbolics]

Thanks, fixed.