Mention swap is not recommended on ref arch

What does this MR do?

We seem to have 2 pages with opinions on swap:

• On https://docs.gitlab.com/ee/install/requirements.html#memory, we recommend that 2GB of swap is added to servers
• On https://docs.gitlab.com/ee/administration/reference_architectures/#no-swap, we recommend that no swap is added

Some users on reference architectures may discover the first link and get confused. Let's reference the second link from the first.

Related issues

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