Add geo hashed storage warning

Merged James Ramsay (ex-GitLab) requested to merge jramsay-geo-hashed-storage-docs into master

What does this MR do?

We are not recommending customers enable hashed storage while issues with hashed storage are resolved.

https://gitlab.com/gitlab-com/infrastructure/issues/2821#note_47753038

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Edited by 🤖 GitLab Bot 🤖