docs(yandex): fix Lockbox referent cell and add Certificate Manager rows (#6585)

Both Yandex providers share the ydxcommon provider, whose NewClient
resolves credentials via resolvers.SecretKeyRef with no secret-selector
validation in ValidateStore and no namespace guard, so a namespace-less
ClusterSecretStore reference resolves in the ExternalSecret namespace.
Mark Yandex Lockbox referent authentication accordingly.

Yandex Certificate Manager has its own doc page and nav entry but was
missing from both the stability table and the feature matrix. Add it
with the same capability profile as Yandex Lockbox (referent
authentication and store validation supported, read-only otherwise).

Signed-off-by: Alexander Chernov <alexander@chernov.it>
Co-authored-by: Gergely Bräutigam <gergely.brautigam@sap.com>

docs/introduction/stability-support.md

@@ -73,6 +73,7 @@ The following table describes the stability level of each provider and who's res
 | [IBM Cloud Secrets Manager](https://external-secrets.io/latest/provider/ibm-secrets-manager/)              |    stable | [@IdanAdar](https://github.com/IdanAdar)                                                            |
 | [Kubernetes](https://external-secrets.io/latest/provider/kubernetes)                                       |      beta | [external-secrets](https://github.com/external-secrets)                                             |
 | [Yandex Lockbox](https://external-secrets.io/latest/provider/yandex-lockbox/)                              |     alpha | [@AndreyZamyslov](https://github.com/AndreyZamyslov) [@knelasevero](https://github.com/knelasevero) |
+| [Yandex Certificate Manager](https://external-secrets.io/latest/provider/yandex-certificate-manager/)      |     alpha | [@AndreyZamyslov](https://github.com/AndreyZamyslov) [@knelasevero](https://github.com/knelasevero) |
 | [GitLab Variables](https://external-secrets.io/latest/provider/gitlab-variables/)                          |     alpha | [@Jabray5](https://github.com/Jabray5)                                                              |
 | [Oracle Vault](https://external-secrets.io/latest/provider/oracle-vault)                                   |    stable | [@anders-swanson](https://github.com/anders-swanson)                                                                                    |
 | [Akeyless](https://external-secrets.io/latest/provider/akeyless)                                           |    stable | [external-secrets](https://github.com/external-secrets)                                             |
@@ -113,7 +114,8 @@ The following table show the support for features across different providers.
 | Azure Keyvault            |      x       |      x       |          x           |            x            |        x         |      x      |              x              |
 | Kubernetes                |      x       |      x       |          x           |            x            |        x         |      x      |              x              |
 | IBM Cloud Secrets Manager |      x       |              |          x           |                         |        x         |             |                             |
-| Yandex Lockbox            |              |              |                      |                         |        x         |             |                             |
+| Yandex Lockbox            |              |              |                      |            x            |        x         |             |                             |
+| Yandex Certificate Manager |              |              |                      |            x            |        x         |             |                             |
 | GitLab Variables          |      x       |      x       |                      |                         |        x         |             |                             |
 | Oracle Vault              |      x       |      x       |                      |                         |        x         |      x      |              x              |
 | Akeyless                  |      x       |      x       |                      |            x            |        x         |      x      |              x              |