Signing docs are stale

[woodruffw]

From #1363:

Please could you also update the docs to reflect these changes?

For example, https://sigstore.github.io/sigstore-python/api/sign/ still refers to the deleted sigstore.oidc.Issuer.production():

Python 3.14.0 (v3.14.0:ebf955df7a8, Oct  7 2025, 08:20:14) [Clang 16.0.0 (clang-1600.0.26.6)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sigstore
>>> sigstore.__version__
'4.0.0'
>>> from sigstore.oidc import Issuer
>>> issuer = Issuer.production()
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    issuer = Issuer.production()
             ^^^^^^^^^^^^^^^^^
AttributeError: type object 'Issuer' has no attribute 'production'

Originally posted by @hugovk in #1363 (comment)

[jku]

would be great to turn these into doctests as well... even if we don't solve the mock token problem for local testing they could be made to run on CI only: it's better than not running them at all

[jku]

Hmm, I got as far as making the doctest work... and then realized the docs don't really look that great as doctests.

I'll just fix the example for now