docs: SAML page — add explicit Prerequisites section for group pre-creation

[Holmus]

Gap

The SAML setup page explains the matching mechanism (groups claim values correspond to External IDs of Flagsmith groups) but does not state, as an explicit prerequisite step, that groups must be pre-created in Flagsmith before configuring SAML.

Customers consistently miss this and have to be told during setup conversations. The implicit requirement is in the page but not surfaced as a numbered prerequisite.

Proposed scope

Add a short "Prerequisites" section near the top of the page:

Before configuring SAML, create your groups in Flagsmith and assign each one an External ID that matches the group identifier your identity provider will send in the SAML groups claim. SAML group claims map one-to-one with Flagsmith group External IDs; if a group does not exist in Flagsmith with the matching External ID, the user will not be added to it at login.

Small change, high friction-reduction.