Convert Getting Started guide from PDF to Markdown

[phertyameen]

Location

docs/getting-started.md

Problem

README links to a getting-started.pdf on a docs branch; this is harder to keep in sync with code changes, search, or maintain than a versioned markdown file alongside the code it documents.

Acceptance Criteria

• Markdown version covers the same content as the current PDF
• Linked from README in place of (or alongside) the PDF reference
• Verified against the actual current installation steps, not the potentially-stale PDF content