Improve new contributor onboarding in README and CONTRIBUTING

[pranjal2004838]

Summary

This PR improves the onboarding experience for first-time contributors by
making the project purpose clearer and adding a short, actionable path to
getting started.

The goal is to reduce initial cognitive load before contributors dive into the
full documentation.

Motivation

While reading the README and CONTRIBUTING guide as a new contributor, I found
that:

• The README explains what Zulip is very well, but doesn’t clearly answer
  “what should I do first?” for contributors.
• The CONTRIBUTING guide is thorough, but can feel overwhelming without a brief
  high-level entry point.

This PR adds lightweight structure to address those gaps without changing any
existing contribution policies.

What’s changed

README

• Added a short “What is this?” section to clarify project purpose
• Added a 5-minute quick start with a simple, linear setup flow
• Included common commands and a brief project structure overview
• Added clear pointers to next steps and contributor documentation

CONTRIBUTING

• Added a step-by-step outline of the contribution flow from fork to merged PR
• Clarified different types of contributions (including documentation)
• Added a small FAQ addressing common beginner questions
• Linked to deeper documentation for contributors who want more detail

Scope and intent

• Documentation-only changes
• No changes to contribution requirements or review process
• Focused on clarity and approachability for new contributors

Testing

N/A (documentation-only changes)