Move GPG signed commits docs to new location

app/views/profiles/gpg_keys/index.html.haml

app/views/projects/commit/_signature_badge.html.haml

doc/README.md

doc/user/project/gpg_signed_commits/index.md

...igned_commits/img/profile_settings_gpg_keys_paste_pub.png

...gned_commits/img/profile_settings_gpg_keys_single_key.png

...igned_commits/img/project_signed_and_unsigned_commits.png

...ommits/img/project_signed_commit_unverified_signature.png

..._commits/img/project_signed_commit_verified_signature.png

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

doc/user/project/repository/index.md