docs: Add Introduction Section

[longfeiWan9]

Closes #322

This PR introduces a new "Introduction" section to the documentation with two key pages that provide a comprehensive overview of Filecoin Onchain Cloud (FOC) for new users and developers.

Changes Made

📄 New Pages Added

• intro/about.md - Comprehensive overview of what Filecoin Onchain Cloud is, its architecture, and core components
• intro/why.md - Detailed explanation of why developers should choose FOC over traditional cloud solutions

🏗️ Re-structure the content for coming content, includes:

• Quick Start
• Concepts
• Developer Guides

[timfong888]

I made some suggestions, none earth shattering but for clarity and priority:

1. Payment Rails: when I read it, the flexibility sounds like I, as the developer, can pay flexibility for the warm storage service; even if we could, I don't think that's what they want, it adds cognitive load. My understanding is they can set up rails for services they offer to their clients, so it's like a way to enable billing to customers for their own services.

2. Related to payment rails: a core goal and metric is exposure of their customers to pay directly for the storage as a preferred mechanism. If we could nudge that way in the text and example.

3. The rest are minor organizational and text changes.

[longfeiWan9]

@timfong888 Thank you for the review. I addressed your comments with some small changes.

Overall, this PR adds an intro section for POC.

• We will use Quick Start for developers to learn how to use Synapse quickly
• How developers will use Rails to pay - this will be covered in the developer guide section.
• Flexible payment models mean developers can pay for Filecoin warm storage more flexibly from the Filecoin pay side. For example, fix rate, pay as you go, etc. We do not consider how the developers will bill their customers.

Let's focus on the "about" and "why" of FOC content to ensure we are explaining FOC correctly and excitingly.

[longfeiWan9]

@rvagg Why is this PR being closed ?

[rjan90]

The PR here: #344 marked "Closes 331", so when it got merged GH automatically closed this.

But the number was a typo, will reopen