docs: Add contributor setup requirements and local workflow for the documentation-first phase

[Sir-Emress]

Problem

The BOS repository has no setup instructions. Since BOS is currently a documentation-first
project, contributors need guidance on what tools they need and how to work with markdown
files locally — not application build steps that don't yet exist.

Proposed Solution

Add a "Local Setup" section to README.md or CONTRIBUTING.md covering:

• Prerequisites (Git, a text editor, a GitHub account)
• How to fork and clone the repository
• How to preview markdown locally (optional: VS Code, Typora)
• How to submit a documentation contribution via PR

Why It Matters

Without any setup instructions, first-time contributors — especially students new to
open source — don't know where to start. Even a minimal "here's what you need" section
reduces drop-off significantly.

Notes

Setup instructions should match the current state of the repository. They should not
reference application runtimes (Java, Node, Python) since BOS does not have runnable
application code at this stage.

Acceptance Criteria

• Prerequisites list is accurate for a documentation-only contribution
• Fork, clone, and PR workflow is documented
• Instructions are beginner-friendly and do not invent build steps
• Content matches the actual current state of the BOS repository