Add External Production Gitaly details to Chart Prerequisites

What does this MR do?

The MR adds a section to the page titled "GitLab Chart Prerequisites" noting the prerequisite of an external Gitaly instance for Production GitLab deployments.

The same requirement is mentioned on the Installing GitLab by using Helm page for Redis, Postgres, and Gitlay. The requirement is mentioned again for Redis and Postgres on the GitLab Chart Prerequisites page but left out for Gitaly. I found this confusing and incongruent on my first read of the docs and wanted to help clarify for future users.

Related issues

N/A

Author checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, follow the instructions in the "Reviewer Roulette" section of the Danger Bot MR comment, as per the Distribution experimental MR workflow

For merge requests from forks, consider the following options for Danger to work properly:

• Consider using our community forks instead of forking your own project. These community forks have the GitLab API token preconfigured.
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added/updated
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Equivalent MR/issue for Gitlab Operator project opened (see Operator documentation on impact of Charts changes)
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.