docs: expand "Why use LangChain?" section in README

[repeat-Q]

Description: Expanded the "Why use LangChain?" section in the main README to provide a more comprehensive overview of the framework's value proposition. Added new subsections covering rapid prototyping capabilities, production-ready features, community and ecosystem benefits, and flexible abstraction layers.

Fixes #33783

Issue: #33783

Dependencies: None

[repeat-Q]

Hi @langchain-ai/maintainers, hope you are having a productive week.

I wanted to kindly bring this documentation PR to your attention and provide some context on the intent behind it.

Background:
After filing issue #33783, I noticed several community discussions where new users were seeking a clearer, more comprehensive value proposition for LangChain. The current "Why use LangChain?" section is a great start, but I believe we can make the onboarding experience even better by highlighting a few more key strengths.

This PR aims to:

Accelerate evaluation for developers by showcasing LangChain's full capabilities upfront.

Reduce onboarding friction by answering common "why" questions directly in the README.

Highlight hidden gems like the vibrant community and production-ready tooling that users often discover later.

The additions (Rapid Prototyping, Production Features, Community, Flexible Abstractions) are based on my own experience and observed community feedback. I've tried to keep the tone and style consistent with the existing content.

I would be truly grateful if you could find a moment to review this. I'm more than happy to incorporate any feedback or adjustments you might have.

Thank you for your time and for building such an amazing project!