Fix broken links to removed performance-indicators page

## Monitor Merge Request Trends

Open merge requests sometimes become idle (not updated by a human in more than a month). Once a month, engineering managers will receive an [`Merge requests requiring attention triage issue`](/handbook/engineering/infrastructure-platforms/developer-experience/triage-operations/#group-level-merge-requests-that-may-need-attention) that includes all (non-WIP/Draft) MRs for their group and use it to determine if any action should be taken (such as nudging the author/reviewer/maintainer). This assists in getting merge requests merged in a reasonable amount of time which we track with the [Open MR Review Time (OMRT)](/handbook/engineering/development/performance-indicators/#open-mr-review-time-omrt) and [Open MR Age (OMA)](/handbook/engineering/development/performance-indicators/#open-mr-age-oma) performance indicators.

Open merge requests sometimes become idle (not updated by a human in more than a month). Once a month, engineering managers will receive an [`Merge requests requiring attention triage issue`](/handbook/engineering/infrastructure-platforms/developer-experience/triage-operations/#group-level-merge-requests-that-may-need-attention) that includes all (non-WIP/Draft) MRs for their group and use it to determine if any action should be taken (such as nudging the author/reviewer/maintainer). This assists in getting merge requests merged in a reasonable amount of time which we track with the Open MR Review Time (OMRT) and Open MR Age (OMA) performance indicators in internal Tableau dashboards.

Open merge requests may also have other properties that indicate that the engineering manager should research them and potentially take action to improve efficiency. One key property is the number of threads, which, when high, may indicate a need to update the plan for the MR or that a synchronous discussion should be considered. Another property is the number of pipelines, which, when high, may indicate a need to revisit the plan for the MR. These metrics are not yet included in an automatically created a triage issue.