Merge DebugCounter documentation

[0xdaryl]

There are two versions of DebugCounter documentation in the compiler:

1. https://github.com/eclipse-omr/omr/blob/master/compiler/ras/DebugCounter.md
2. https://github.com/eclipse-omr/omr/blob/master/doc/compiler/debug/DebugCounters.md

Both are obviously derived from the same root document. They should be compared for differences and merged.

I think version 2) should be the lone document going forward because it is in the better spot and seems to be formatted better already. Version 1) was the original contribution and did undergo a review with comments (#340), so part of this work is ensuring those minimal comments are captured in 2). Afterwards, 1) should be deleted.