docs(self-host): surface embedded IdP sync docs in sidebar

[SunsetDrifter]

Summary

• Add the embedded IdP sync pages (Microsoft Entra ID API/SCIM, Google Workspace, JumpCloud, Keycloak) to the Self-Host NetBird → Authentication sidebar so they're discoverable instead of only reachable by direct URL.
• Regroup the Authentication subsection by purpose: Local Users, Identity Provider Connectors (wraps the existing Self-hosted and Managed IdP groups), and Provision Users & Groups (new).
• The existing MANAGE NETBIRD → Provision Users & Groups entries (cloud variants) are unchanged.

Before / after

Authentication                          Authentication
  Authentication and IdPs       →         Authentication and IdPs
  Local User Management                   Local Users
  Disable Local Auth                        Local User Management
  Self-hosted IdPs                          Disable Local Auth
    Generic OIDC, Zitadel, …              Identity Provider Connectors
  Managed IdPs                              Self-hosted IdPs
    Google Workspace, …                       Generic OIDC, Zitadel, …
                                            Managed IdPs
                                              Google Workspace, …
                                          Provision Users & Groups   ← NEW
                                            Microsoft Entra ID (API)
                                            Microsoft Entra ID (SCIM)
                                            Google Workspace
                                            JumpCloud
                                            Keycloak