Monorepo Guide (1st iteration)

What does this MR do and why?

Add a monorepo guide in our documentation to use with customers who are onboarding with GitLab, to serve as both a guide and a warning to those with certain repository structures and access patterns that could lead to performance issues.

fixes: gitaly#5320 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.