OM2: docs: Document OpenMetrics 2.0 preview and OTel preserve_names flag

[zeitlinger]

Part of #1912.

Add documentation for two new preview features:

OpenMetrics 2.0 (preview)

• io.prometheus.openmetrics2.enabled=true activates the OM2 writer
  (feat: Add OpenMetrics2 enabled flag #1953)
• OM2 outputs metric names as provided — no _total or unit suffix
  appending (unlike OM1)
• Feature flags: contentNegotiation, compositeValues,
  exemplarCompliance, nativeHistograms
• Programmatic: enableOpenMetrics2(om2 -> om2.contentNegotiation(true))

OTel preserve_names flag

• io.prometheus.exporter.opentelemetry.preserve_names=true stops
  stripping _total and unit suffixes when pushing to OTel
• Default false (legacy behavior) — will flip to true in next
  major release (OTel exporter: preserve_names=true as default #1943)

Where to document

• docs/content/getting-started/ or a new
  docs/content/openmetrics2/ section
• Config reference page