update(config): Add more samples for Dashboard config

[Meggielqk]

The customer support is asking for more config item explanations and samples in Dashboard configuration doc.

[id]

We can add here that by default it's 60 minutes.
May be worth clarifying that this parameter is equivalent to "browser session expiration time". When user login to EMQX Dashboard, EMQX generates a JWT token which is stored as a cookie in browser along with a refresh token. While the session is active, dashboard will be refreshing the session token when it's close to expiration.

[id]

Example

sso = {
  ldap = {
    enable = true
    backend = "ldap"
    query_timeout = "5s"
    server = "localhost:389"
    pool_size = 8
    username = "cn=admin,dc=example,dc=com"
    password = "secret"
    base_dn = "dc=example,dc=com"
    filter = "(& (objectClass=person) (uid=${username}))"
    request_timeout = "10s"
  }
  oidc = {
    enable = true
    backend = oidc
    issuer = "https://issuer.example.com"
    clientid = "your-client-id"
    secret = "your-client-secret"
    scopes = [
      "openid"
    ]
    name_var = "${sub}"
    dashboard_addr = "http://127.0.0.1:18083"
    session_expiry = "30s"
    require_pkce = false
    preferred_auth_methods = [
      "client_secret_post",
      "client_secret_basic",
      "none"
    ]
    provider = generic
    fallback_methods = [
      "RS256"
    ]
  }
  saml = {
    enable = true
    backend = "saml"
    dashboard_addr = "https://127.0.0.1:18083"
    idp_metadata_url = "https://idp.example.com"
    sp_sign_request = false
    sp_public_key = "Pub Key"
    sp_private_key = "SP Private Key"
  }
}

[id]

Normally only one of ldap, oidc or saml will be active at the same time. Here we list all of them just for the illustration purposes.