replace hardcoded urls to docs.pytest.org with internal crossrefs, add external crossrefs where possible

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>

doc/en/conf.py

@@ -123,7 +123,6 @@ pygments_style = "sphinx"
 # A list of regular expressions that match URIs that should not be checked when
 # doing a linkcheck.
 linkcheck_ignore = [
-    "https://github.com/numpy/numpy/blob/master/doc/release/1.16.0-notes.rst#new-deprecations",
     "https://blogs.msdn.microsoft.com/bharry/2017/06/28/testing-in-a-cloud-delivery-cadence/",
     "http://pythontesting.net/framework/pytest-introduction/",
     r"https://github.com/pytest-dev/pytest/issues/\d+",
@@ -350,6 +349,7 @@ texinfo_documents = [
 intersphinx_mapping = {
     "pluggy": ("https://pluggy.readthedocs.io/en/stable", None),
     "python": ("https://docs.python.org/3", None),
+    "numpy": ("https://numpy.org/doc/stable", None),
 }