[feat]: Improve RustShare README first-screen explanation

[melisecolak]

Problem

Context

The first screen of the README is the first thing launch visitors see when arriving from LinkedIn, X, GitHub search, or community posts.

We need to make sure the core positioning is clear, accurate, and honest about the current Public Preview status.

Task

Review the top section of README.md and improve the header and introductory text so first-time visitors can quickly understand what RustShare is.

The wording should align with this approved direction:

# RustShare

**Open-source company memory workspace for durable team knowledge, files, notes, documents, and future permission-aware AI workflows.**

> [!NOTE]
> RustShare is currently in **Public Preview**. The project is early, actively evolving, and not yet intended for production use. We are opening the project for feedback, testing, optional GitHub stars, and first contributors.

---

[Quickstart](#quickstart) • [Contributing](#contributing) • [Beta Tester Program](#beta-tester-program) • [License](#license)

### Proposed Solution

Update the top section of `README.md` so the first screen clearly explains RustShare and its Public Preview status.

The wording should align with this approved direction:

```md
# RustShare

**Open-source company memory workspace for durable team knowledge, files, notes, documents, and future permission-aware AI workflows.**

> [!NOTE]
> RustShare is currently in **Public Preview**. The project is early, actively evolving, and not yet intended for production use. We are opening the project for feedback, testing, optional GitHub stars, and first contributors.

---

[Quickstart](#quickstart) • [Contributing](#contributing) • [Beta Tester Program](#beta-tester-program) • [License](#license)

Suggested area:

README.md, top section.

Expected outcome:

Visitors should immediately understand that RustShare is an open-source company memory workspace in Public Preview.

Acceptance criteria:

• The first screen clearly explains what RustShare is.
• Public Preview wording is visible.
• Navigation links to Quickstart, Contributing, Beta Tester Program, and License are present if those sections exist.
• No production-ready or enterprise-ready claims are added.
• Wording does not imply that future AI/RAG features are already complete.

Alternatives Considered

Leave the README as-is, but that may make the first launch visitors work harder to understand the product positioning and Public Preview status.

Additional Context

This is intended as a good first issue for documentation-focused contributors. No Rust compiler, database setup, or local application setup is required.

Checks

• I have searched existing issues and this feature has not been requested yet.