DEF-RENDER-HOOK: Store symbols on *RENDER-HOOK*.

PUSHing the functions themselves means that the hooks get installed
every time the DEF-RENDER-HOOK forms get evaluated, even if they
already exist.
Using PUSHNEW and the symbols avoid that.

To reproduce, just
  (load "build.lisp")
  touch cl-site.asd # force asdf to reload the system
  (load "build.lisp")
and the output will show multiple identical lines - once
for each time the system got loaded.
  Transforming libraries.md to libraries.html
  Transforming libraries.md to libraries.html
  Transforming libraries.md to libraries.html
1 job for only-transform-once in 3 seconds (queued for 40 seconds)
Status Job ID Name Coverage
  Build
passed #1112
site-gen
build

00:00:03