Review signed commits page

...igned_commits/img/project_signed_and_unsigned_commits.png

...commits/img/project_signed_and_unsigned_commits_v17_4.png

...ommits/img/project_signed_commit_unverified_signature.png

.../img/project_signed_commit_unverified_signature_v17_4.png

..._commits/img/project_signed_commit_verified_signature.png

...ts/img/project_signed_commit_verified_signature_v17_4.png

doc/user/project/repository/signed_commits/index.md