TINYDOC-3593: Correct the AI license key environment variable spelling and flag the license key placeholder.

[kemister85]

Ticket: TINYDOC-3593

Site: Staging branch

Changes:

modules/ROOT/pages/tinymceai-on-premises-getting-started.adoc

• Renamed all five occurrences of the .env alias AI_LICENCE_KEY to AI_LICENSE_KEY for consistent US English spelling, across the .env example, the [IMPORTANT] admonition, the prose describing the environment variable mapping, the docker run block, and the [WARNING] admonition. The placeholder value on the same line was also normalized (paste-ai-licence-key-here to paste-ai-license-key-here).
• Left LICENSE_KEY (the container's own environment variable) and license_key (the tinymce.init() option) unchanged, as both are fixed API surface. The [IMPORTANT] admonition explaining that these are separate, non-interchangeable credentials is unchanged apart from the renamed alias.
• Added a NOTE admonition after the token-server.js listing instructing the reader to replace 'your-license-key' with the editor license key, distinguishing it from the AI service license key and linking to the License key page. The listing itself is unmodified.
• Corrected the anchor on two cross-references to the Credentials section of the on-premises overview page. Both pointed at #_credentials, which does not exist on that page; the correct anchor is #credentials. One of these was pre-existing and produced a broken link.

Pre-checks:

• Branch is correctly prefixed.
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.