[DOC] Improve Concepts welcome page

[s-n-park]

Is your feature request related to a problem? Please describe.
User submitted feedback of "missing-info" tag on https://docs.near.org/concepts/welcome
"What is NEAR? What's the consensus mechanism? Who runs nodes? This page is a mess. Way too many clicks required to see basic info."

Describe the solution you'd like
User should get a clear and concise explanation of NEAR that can walk a beginner through

Additional context
https://panel.feelback.dev/projects/03925b69-72cd-4bc8-b26e-bcf88f89e542/feelbacks/04510ef1-df33-437b-aa0c-caf7c993ce9a