docs: Clarify DCO identity guidance

The DCO policy used here appears to have been derived from the Linux
kernel's guidance. In 2023, that guidance changed to relax the "real
name" requirement to a "known identity".

The DCO sign-off identifies the person certifying that they have the
right to submit a contribution. The previous wording required a "real
name" and rejected pseudonyms, which could discourage contributors
whose preferred or community name is the identity by which they can be
known.

The requirement is not anonymity: the identity in the sign-off must be
known enough for the project to follow up about the contribution if a
question arises. It does not require a legal name or a government-issued
identity. Use the clarified wording from the Linux kernel so known
pseudonyms and preferred names remain acceptable while anonymous
contributions remain excluded.

This also reflects concerns raised in the CNCF community about whether
"real name" means a legal name, a government-issued identity, or a
public identity that is sufficient to identify and contact a
contributor. The discussion emphasized that attribution and future
accountability do not require publishing a legal identity, and that
strict public-name rules can exclude people who cannot safely contribute
under that name.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d4563201f33a022fc035

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

CONTRIBUTING.md

@@ -267,7 +267,7 @@ Then you just add a line to every git commit message:
 
     Signed-off-by: Joe Smith <joe.smith@email.com>
 
-Use your real name (sorry, no pseudonyms or anonymous contributions.)
+Use a known identity (sorry, no anonymous contributions.)
 
 If you set your `user.name` and `user.email` git configs, you can sign your
 commit automatically with `git commit -s`.