[IMP] Helpdesk: merge tickets

[jero-odoo]

Adding a section on the Helpdesk basics doc about how to merge tickets, and linking to new data cleaning app documentation

Docs task: https://www.odoo.com/mail/view?model=project.task&res_id=4029119&access_token=b3121ca8-2a87-49d3-a08d-d1f352589686

[robodoo]

[jero-odoo]

@larm-odoo This is ready for review. Thanks!

[jero-odoo]

Thanks Lara! @ksc-odoo this is ready for final review. Thanks!

[jero-odoo]

Thanks KC! Hey @samueljlieber this is ready for tech review. Thanks!

[jero-odoo]

@robodoo r+