Add more Monitor pages to left menu

Now that these stub pages exist, add them to the left navigation:

• /operations
• /operations/metrics
• /operations/metrics/embed.html
• /operations/metrics/embed_grafana.html
• /operations/metrics/dashboards
• /operations/metrics/dashboards/panel_types.html
• /operations/metrics/dashboards/yaml.html

and update the following moved pages:

• /operations/feature_flags.html
• /operations/tracing.html

Related to gitlab#227441 (closed) which charts out the overarching IA work this MR is a part of.