Add script to launch livereload server for docs
Add targets buildhtml and livehtml to docs Makefile

The former is required to avoid an infinite rebuild loop when the
livereload server is running: it is watch source to rebuild the docs
but also firedrake to rebuild apidoc. However the html make target
first invokes apidoc and thereby triggers an infinite loop. Hence add
a target buildhtml that does not depend on apidoc.