DOC: config: move stick-tables and peers to their own section

As suggested by Tim in issue #2953, stick-tables really deserve their own
section to explain the configuration. And peers have to move there as well
since they're totally dedicated to stick-tables.

Now we introduce a new section "Stick-tables and Peers", explaining the
concepts, and under which there is one subsection for stick-tables
configuration and one for the peers (which mostly keeps the existing
peers section).

Author: wtarreau