docs: move most docs to the wiki

I've moved the installation and developer guides, FAQ and how-tos from
hledger.org (yst & git) to hledger.org/wiki (dokuwiki) and tried to
integrate their navigation bars two as best I can. Using just one
or the other would be nice, but I don't think I can quite do that;
it seems the manual at least should be revision controlled along with the code.