Tighten Streams page openings and headings

[Copilot]

This updates the Streams docs pages flagged for weak openings so each page states its purpose up front and uses more specific, context-rich headings. The affected pages now read as Streams-specific entry points instead of generic feature pages.

• Scope

  • Updated the main Streams landing page plus the flagged management pages under solutions/observability/streams/
  • Focused on H1 clarity and opening paragraph strength only

• Heading updates

  • Made generic titles explicit about Streams context
  • Kept existing anchors where needed by preserving or updating page IDs

• Opening paragraph rewrites

  • Expanded weak intros into 2–4 sentence openers that explain what the page covers and why it matters
  • Added direct guidance for page purpose and navigation context

Example:

# Manage data streams in Kibana [streams]

Streams provides a single, centralized UI within {{kib}} that streamlines common tasks like extracting fields, setting data retention, and routing data.