[ADD] events: registration desk doc (17)

[ksc-odoo]

PROJECT TASK: https://www.odoo.com/web#id=3931473&cids=3&menu_id=4720&action=333&active_id=3835&model=project.task&view_type=form

This concise PR is focused on briefly explaining how to grant attendees access to an event using the Odoo Events 'Registration Desk' feature.

• explains the functionality of the 'Registration Desk' feature in the Odoo Events application.
• adds 'registration_desk' to the toctree

[robodoo]

Pull request status dashboard.

[ksc-odoo]

Hey @Felicious -- this one is ready for a Peer Review, whenever you get a chance. Thanks! 🙏

[ksc-odoo]

Thanks for the always helpful feedback, @Felicious -- I implemented your suggested changes/adjustments, and believe this one is now ready for a final review from @StraubCreative (whenever you get a chance) Thanks! 👍

[StraubCreative]

Fix merge conflict on ca74b96

[ksc-odoo]

@robodoo r+