Document that get_template() template names should use / even on Windows

[simonw]

I got caught out by this recently. The only mention I could find of this was in this comment: #1378 (comment)

Jinja template names/paths are not always filesystem paths. So regardless of the OS Jinja always uses forward slashes.

[simonw]

I think the best place to document it is here:

jinja/src/jinja2/environment.py

Lines 965 to 976 in 077b791

	@internalcode
	def get_template(
	self,
	name: t.Union[str, "Template"],
	parent: t.Optional[str] = None,
	globals: t.Optional[t.MutableMapping[str, t.Any]] = None,
	) -> "Template":
	"""Load a template by name with :attr:`loader` and return a
	:class:`Template`. If the template does not exist a
	:exc:`TemplateNotFound` exception is raised.
	:param name: Name of the template to load.