[ADD] rental: add customer signature page

[meval1006]

Update the section on the Request a Signature feature to redirect steps to the Sign doc. Specifically, the Use Templates > From a record section: https://www.odoo.com/documentation/19.0/applications/productivity/sign/request_signatures.html#sign-templates-use-odoo-record

• Rearranged the setting configuration to be in the Customer Signature section.
• Keep the context of what a rental and service agreement does.

This 19.0 PR should be FWP to master.

Task: https://www.odoo.com/odoo/action-4043/5883421

[robodoo]

[meval1006]

@Felicious This is ready for review. I will create separate PRs to update the existing Customer Signature sections on the Product and Service pages.