docs: sign digest header in hmac-auth example

[leno23]

Description

Update the hmac-auth body-validation example so the generated HMAC signature also covers the Digest header:

• compute the request body digest before building the signing string
• add digest: ... to the signing string
• include digest in headers="@request-target date digest"
• refresh the printed header, curl, and response examples
• add a short note explaining that validate_request_body validates the body against Digest, and signing digest binds that digest to the HMAC signature

Fixes #13395

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible

Validation

• git diff --check
• extracted the documented Python snippet, fixed the timestamp to the sample output value, and verified it prints:
  • Digest: SHA-256=78qzJuLwSpZ8HacsTdFCQJWxzPMOf8bYctRk2ySLpS8=
  • headers="@request-target date digest"
  • signature="LGBTz7bVQQWlkijeyDpEwJWo+ppwX735uRZk5F8KhmU="