Update Gofity docs

[michielkalle]

See: https://github.com/n8n-io/n8n/pull/23915

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Summary by cubic

Added a "Create message" section to the Gotify node docs to clarify how to configure messages. Covers priority, title, content type (plain/markdown), and message extras (click URL, big image URL, intent URL), with a link to Gotify’s extras docs.

^{Written for commit 56cbdb6b2cf2c9570ec8e5f2a5b6b37407fce811. Summary will update on new commits.}

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[github-actions[bot]]

Thanks @michielkalle! We'll hold off on reviewing this until the PR is ready to go in the main repo.