[IMP] l10n_gt: Add Infile registration form link to documentation

[fw-bot]

This PR updates the Guatemala EDI documentation to include a direct link to the official Infile registration form used by Odoo customers to request their account.

Changes included

• Added a new note under the Infile subsection with the registration form link and a short explanation of how Infile will contact the user to complete the onboarding.
• Added a reference to the same form in the Introduction section to give users an additional entry point when starting the setup.

Purpose

This update helps new customers begin their Infile onboarding process more easily by providing the correct registration channel directly in the documentation. It ensures that users know where to submit their information before configuring credentials in Odoo.

No functional changes

Documentation-only update.

Forward-Port-Of: #15291

[robodoo]

[fw-bot]

@fvz-odoo @afma-odoo this PR targets master and is the last of the forward-port chain.

To merge the full chain, use

@robodoo r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port