Fix moved help URL for administration monitoring performance

What does this MR do?

Fix moved help URL for administration monitoring performance

Gitlab.com as example:

Before: https://gitlab.com/help/administration/monitoring/performance/introduction

After: https://gitlab.com/help/administration/monitoring/performance/index.md

Screenshots

Moved link

Target correct link

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides
• Separation of EE specific content