Add explanatory note at the top of the docs page

[oubiwann]

We've received some feed back on Twitter about the lack of a good LFE tutorial; that's got a new ticket (issue #40).

While that work is in progress, we need to do some "preventative medicine" by clearly setting expectations (a little of this was done in the README for Casting SPELs in LFE; we need a shorter version of that for our current docs).

So, at the top of the docs site, we need to do the following:

• Convert the section "Erlang Resources" to "Learning LFE"
• Add explanatory text saying that, until we have more and better beginners' materials, one needs to learn a bit of Lisp and some Erlang before tackling LFE
• Provide resources for both, with links
• Provide a note on "what's next", i.e., once that task has been accomplished, what the reader can do next

Both Quick Starts should have their introductions updated along these lines as well.