docs: Should mention the forum post template for applying for classic confinement

Specifically the following link:

[About the classic-confinement category - store-requests / classic-confinement - snapcraft.io](https://forum.snapcraft.io/t/about-the-classic-confinement-category/43830)


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*Reported from: https://snapcraft.io/docs/reference/administration/reviewing-classic-confinement-snaps/*