Add Pretix link to register for Welcome Session

[sheraaronhurt]

responding to #1938 (comment)

[github-actions]

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⏱️ Updated at 2025-12-17 20:21:57 +0000

[brownsarahm]

this should also be updated on the checkout page.

[ndporter]

this should also be updated on the checkout page.

This was resolved by #1952 and #1955 but the language in this PR is still a little better now the conflict is resolved. Can you mark the changes approved @brownsarahm so it can be merged?