docs: state that the License Manager uses Timefold Platform credentials

[oemebamo]

Closes TimefoldAI/timefold-solver-enterprise#920

The issue is tracked in timefold-solver-enterprise, but the License management page lives in this repo.

Changes (commercial-editions/license-management.adoc)

• Intro: added that you log in to the License Manager (licenses.timefold.ai) with your Timefold Platform credentials, so you don't need a separate login.
• Register and free trial, step 2: added that existing Timefold Platform users log in with those credentials, with a link to Register on Timefold Platform for anyone who doesn't have credentials yet.

Review notes

• The issue says "no separate account". I wrote "login" instead, because the terminology rules avoid using "account" for a single person's credentials.
• The page doesn't say whether existing Platform users can skip the License Manager registration form (company details, terms of service). That wasn't part of the confirmed facts, so it's worth checking.

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