Strengthen frontmatter descriptions across 13 reference pages

[Copilot]

This updates 13 reference pages whose frontmatter description fields were weak, title-like fragments. The new descriptions are concise, value-focused sentences that align with the docs frontmatter quality requirements.

• Scope

  • Updated frontmatter descriptions in:
    • docs/reference/index.md
    • docs/reference/central-configuration.md
    • docs/reference/edot-cloud-forwarder/index.md
    • docs/reference/edot-sdks/index.md
    • docs/reference/motlp/index.md
    • docs/reference/motlp/rate-limiting.md
    • docs/reference/compatibility/index.md
    • docs/reference/compatibility/collectors.md
    • docs/reference/compatibility/edot-vs-upstream.md
    • docs/reference/compatibility/features.md
    • docs/reference/compatibility/limitations.md
    • docs/reference/compatibility/nomenclature.md
    • docs/reference/compatibility/sdks.md

• What changed

  • Replaced label-style openings such as Reference documentation for, Introduction to, and Overview of
  • Rewrote descriptions as complete sentences
  • Shifted wording toward user value and page content rather than page labels
  • Kept each description within the expected short frontmatter form

• Example

  - description: Reference documentation for the Elastic Cloud Managed OTLP Endpoint.
  + description: The Elastic Cloud Managed OTLP Endpoint provides a hosted ingestion layer for sending OpenTelemetry data directly to Elastic Cloud using the OTLP protocol.