jit,docs: use enum directive for enumeral types

jit,docs: use enum directive for enumeral types

gcc/jit/ChangeLog:

	* docs/conf.py: Add needs_sphinx = '3.0' where c:type was added.
	* docs/index.rst: Remove note about it.
	* docs/topics/compilation.rst: Use enum directive and reference.
	* docs/topics/contexts.rst: Likewise.
	* docs/topics/expressions.rst: Likewise.
	* docs/topics/functions.rst: Likewise.