docs: Expand README with project overview, active projects, and contributor onboarding steps

[Sir-Emress]

Problem

The current BOS README is too short to explain what BOS is, what projects it coordinates,
or how a new contributor should get started. First-time visitors have no clear entry point
into the Bugema Open Source Community.

Proposed Solution

Rewrite README.md to include:

• A clear description of BOS as a documentation-first coordination hub for bos-com projects
• A table of active repositories (GreenCode, OpenCare-Core, LifeLine-ICT, etc.) with links
• Step-by-step "Getting Started" section: fork → clone → branch → PR
• Repository structure overview
• Community standards and license reference

Why It Matters

BOS is the first repository a new Bugema Open Source Community contributor encounters.
A clear README sets expectations, reduces onboarding friction, and signals that the
project welcomes contributions.

Acceptance Criteria

• README explains BOS purpose as a documentation-first community hub
• Active project table links to all main bos-com repos with descriptions
• Step-by-step onboarding instructions are accurate and beginner-friendly
• Repository structure section is included
• License and community standards are referenced.