Added grace extension related details.

Why is this change being made?

Based on the discussion here we didn't have the information available in our handbook about how sales can create a grace extension for an existing customer to sort out any hiccups with procurement for renewals. The current allowed grace period is 2 weeks.

For now, we can use the current template of trial extensions but for statistics purposes, we can create a separate template for extension related to existing customers.

Feel free to make any amends in case required.

cc// @vijirao @lyle @ralfaro @gitlab-com/support/dotcom/console

Author Checklist

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