Document new Envoy resource manager functionality

[kralicky]

New memory management features added in this PR: pomerium/pomerium#5106

This system is automatic, but users can disable it using a runtime flag. There are no other user-adjustable options. It would be helpful to note that this system exists and how it interacts with pod resource limits (it is enabled if you have a memory limit set, otherwise it is disabled).

A list of the current (hard coded for now) memory usage % thresholds at which specific memory-reducing actions are taken is here: https://github.com/pomerium/pomerium/blob/42bb706a9cd0c9828550e6d141e7c63263e0bfad/pkg/envoy/resource_monitor_linux.go#L56-L82

[ZPain8464]

I will start documenting this feature this week, sorry for the delay.