Improve the diagnostics ToC and content organization

[samsp-msft]

Based on conversations with @noahfalk about #35444, we have come up with an improved ToC for the docs area:

Diagnostics and Instrumentation
  Overview
  Debuggers (reworked managed debugging)
  Logging & tracing options (reworked from #35444)
  ILogger Logging (Link to existing docs elsewhere in ToC)
  Metrics (same)
  Distributed tracing (same)
  Specialized Diagnostics
    Overview (updated from EventSource)
    EventSource
          + Runtime Event details (was Runtime Events)
    Diagnostic Port
       Merge DiagnosticPort & client library sections
    Symbols
    Dumps
  .NET Diagnostic Tools
      Overview (new, reworked from #35444)
      ...

@samsp-msft to implement