Improve UX for Adding License Keys in Self-Hosted Instance

[marketachalupnikova]

When users try to add a license key to a self-hosted instance, the process is unclear. In particular, it's not obvious where the Server Administration section is located.

1. Update Documentation

• Add new screenshots to the licensing guide: https://docs.tolgee.io/platform/self_hosting/licensing
• Clearly describe where to find the Server Administration section in the UI of the selfhosted instance

2. Enhance In-App UI

• In the license key input section, mention where Server Administration is located
• Add a link to the documentation page