docs: unify getting started content into first steps (#3066)

[santhiprakash]

Fixes #3066

Problem

tutorials/getting_started and introduction/first_steps heavily overlap: both walk through downloading/configuring/running Prometheus. Per @jan--f's correction on #3069, the direction is now to unify the content in introduction/first_steps and remove tutorials/getting_started.

Triage / what changed

• Folded the tutorial's unique hands-on piece (download → run node exporter → add a second scrape_configs job → query a host metric) into first_steps' existing "Monitoring other targets" section, keeping the deeper node exporter guide link.
• Added an early pointer to the Overview for readers who want concepts/architecture before hands-on (the conceptual sections of the tutorial — "what is Prometheus", metrics basics, architecture — were already covered there and in the glossary).
• Removed docs/tutorials/getting_started.md and its three images, which nothing else referenced (architecture.png, sample_graph.png, node_exporter.png under public/assets/docs/tutorial/).
• Added /docs/tutorials/getting_started/ → /docs/introduction/first_steps/ to public/_redirects, matching the file's existing format.
• Updated the Summary to reflect the node exporter step.

Verification

• make check (mdox fmt --check) passes.
• Every link in the rewritten section verified against the tree or existing generated routes: /download#node_exporter matches the existing pattern in docs/guides/node-exporter.md:16; overview, node-exporter, and exporters pages exist.
• grep confirms no remaining references to tutorials/getting_started or the removed images anywhere in the repo.

Notes

• The two YouTube embeds in the old tutorial were not carried over (they duplicate the on-page steps); happy to re-add either one to first_steps if you'd like to keep them.
• @jan--f you also mentioned unifying https://prometheus.io/docs/prometheus/latest/getting_started/ into first steps — that page is sourced from prometheus/prometheus's docs directory, so it needs a cross-repo PR plus _redirects updates (lines 2 and 46 point at it today). Should I proceed with that after this one, or handle it separately?