Deprecate docs.lfe.io in favour of books and lfe.io/docs

[oubiwann]

This will be done in conjunction with the release of the updated LFE site here: https://github.com/lfe/lfe.github.io/issues/113

Additional features/tasks:

• [ ] Ensure all existing docs content exists elsewhere, in linkable form
• [x] https://github.com/lfe/lfe.github.io/issues/120
• [ ] Change docs.lfe.io to redirect to lfe.io/docs
• [ ] Move lfe/docs repo to lfe-deprecated/docs
• [ ] Archive lfe-deprecated/docs