docs: add self-hosted license install and management guide

[wangsijie]

Summary

Add a self-hosted license guide under Logto OSS (/logto-oss/self-hosted-license), covering:

• What a Self-hosted Pro license unlocks: hide Logto branding, Bring your UI and custom CSP, Console members and invitations, mandatory Console MFA, and unlimited SAML apps.
• Getting a license and choosing the production or non-production key.
• Installing a license in Console (Tenant Settings > License) or with PUT /api/systems/license.
• What the license page shows.
• Weekly refresh, the 30-day grace period, and exactly what an instance sends (licenseId, deploymentId, Logto version).
• License expiration, renewal, and what happens to features when the license lapses.
• Storage setup for Bring your UI on self-hosted deployments.

The Logto OSS overview page links to the new guide. Self-serve purchase, payment, invoice, and cancellation instructions are intentionally left out; they ship with the separate purchase project.

The Management API reference is generated from the OpenAPI file in logto-io/logto, which already documents the two /api/systems/license routes, so there is nothing to change here.

The Console license page links to this guide in logto-io/logto#9710.

Testing

Tested locally

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