Add Filesystem Performance Benchmarking documentation

What does this MR do?

Related issues

Filesystem performance can have a big impact on overall GitLab performance, especially for actions that read or write Git repositories. This information will help benchmark filesystem performance against known good and bad real-world systems.

Normally when talking about filesystem performance the biggest concern is with Network Filesystems (NFS). However, even some local disks can have slow IO. The information on this page can be used for either scenario.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards