Update manual to reflect infrastructure changes.

* slime.el (slime-setup-contribs): typo in docstring and missing
newline.

* doc/slime.texi (Platforms): SLIME supports only GNU Emacs 23.4
and 24.4.
(Downloading): Adapt to Git changeover.
(Git): New node adapted from CVS node.
(CVS): Deleted node.
(Git Incantations): New node adapted from CVS node.
(CVS Incantations): Deleted node.
(Installation): Restructure to match README.md recipe.
(Installation): New subsection "Emacs 23 gotchas".
(Autoloading): Restructure to match README.md recipe. Mention
contribs early.
(Setting up pathname translations): Recommend `add-to-list'.
(Loading Contribs): Suggest variable `slime-contribs', not
function `slime-setup'.
(Loading Contribs): New subsection "Loading and unloading on the
fly".
(REPL): Use `add-to-list' instead of `push'.
(slime-autodoc-mode): Use `add-to-lis' instead of `push'.