docs(previder): correct find/referent matrix cells and fix headings (#6577)

The support matrix marked Previder with find by name, but GetAllSecrets is not
implemented, so find does not work; set it blank. It also left referent
authentication blank, but referent works (no strict validator; credentials resolve
via resolvers.SecretKeyRef against the ExternalSecret namespace, same as passbolt);
set it x. Also promote the page headings (H2 title to H1, H3 to H2, H4 to H3).

Signed-off-by: Alexander Chernov <alexander@chernov.it>

docs/introduction/stability-support.md

@@ -134,7 +134,7 @@ The following table show the support for features across different providers.
 | Passbolt                  |      x       |              |                      |                         |        x         |             |                             |
 | Infisical                 |      x       |              |                      |            x            |        x         |      x      |              x              |
 | Bitwarden Secrets Manager |      x       |              |                      |                         |        x         |      x      |              x              |
-| Previder                  |      x       |              |                      |                         |        x         |             |                             |
+| Previder                  |              |              |                      |            x            |        x         |             |                             |
 | Cloud.ru                  |      x       |      x       |                      |            x            |        x         |             |              x              |
 | Volcengine                |              |              |                      |            x            |        x         |             |                             |
 | ngrok                     |              |              |                      |                         |        x         |      x      |                             |

docs/provider/previder.md

@@ -1,17 +1,17 @@
 
 ![Previder Secret Vault](../pictures/previder-provider.png)
 
-## Previder Secret Vault Manager
+# Previder Secret Vault Manager
 
 External Secrets Operator integrates with [Previder Secrets Vault](https://vault.previder.io) for secure secret management.
 
-### Authentication
+## Authentication
 
 We support Access Token authentication using a Secrets Vault ReadWrite or ReadOnly token.
 
 This token can be created with the [vault-cli](https://github.com/previder/vault-cli) using an Environment token which can be acquired via the [Previder Portal](https://portal.previder.nl).
 
-#### Access Token authentication
+### Access Token authentication
 
 To use the access token, first create it as a regular Kubernetes Secret and then associate it with the Previder Secret Store.
 
@@ -40,7 +40,7 @@ spec:
 ```
 
 
-### Creating external secret
+## Creating external secret
 
 To create a kubernetes secret from the Previder Secret Vault, create an ExternalSecret with a reference to a Vault secret.