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: lfe/lfe.github.io#113

Additional features/tasks:

• Ensure all existing docs content exists elsewhere, in linkable form
• Add /docs page lfe.github.io#120
• Change docs.lfe.io to redirect to lfe.io/docs
• Move lfe/docs repo to lfe-deprecated/docs
• Archive lfe-deprecated/docs