Add section to Fulfillment Guide about storage enforcement notifications

Objective: document all the notifications we have going to customers about namespace / project enforcement

Asks:

• Please review changes and verify for accuracy especially where I have (needs to be verified)
• I am going to add in some example images in a future MR. I have most of them but still lacking what CLI notifications might look like for group namespace storage enforcement, so if you have a link for me that would be great.

PS you might be wondering why I am documenting this here. I discussed with Omar and he recommended preferred documented order:

• docs.gitlab.com - transparent to the customer, easily searchable, very official.
• fulfillment guide - public, anyone can find it including Google, kinda official, but messy
• internal handbook - if it really can’t be public
• an issue

Author Checklist

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