docs: improve 'first steps' with data verification and metric discovery

[Atharva-Ramawat]

Fixes #3032

Hi team! This PR addresses the friction points mentioned in issue #3032 for new users going through the "First Steps" guide.

I have added:

• Instructions on verifying data flow using the Status > Targets page.
• Guidance on discovering available metrics using the Metrics Explorer and autocomplete.
• A brief section introducing Recording Rules for saving complex queries, linking out to the full documentation.