Add /docs page

[oubiwann]

Content:

• Note about old docs.lfe.io site pointing here
• Descriptions of "Learn" and "Use" portions of site
• Note about detailed docs being in the books, and where to find what type of documentation
• Add link to /books

[oubiwann]

It's actually already in /static and is a redirect. I'm just going to redirect it from docs.lfe.io to lfe.io/use 😉