Refactor README.md into an approachable Getting Started guide

## Summary

Refactor the README.md into an approachable "Getting Started" guide focused on the Docker workflow. Move detailed/dense content into `docs/` to create a user manual structure, keeping the README as the front door for humans arriving from blog posts.

## Problem

The current README is dense and comprehensive but not approachable. A developer arriving from a blog post should be able to go from zero to indexed repo in minutes. Right now the README has a lot of valuable content but it's hard to find the quick path through it.

## Proposed Approach

### README.md — The front door

- **Focus on the Docker flow** as the easiest path to get started
- Clear, linear steps: set up Elasticsearch → run the Docker image → verify it worked
- Minimal prerequisites, minimal decisions
- Link out to `docs/` for everything else (configuration reference, local dev setup, architecture, advanced options)

### `docs/` — The user manual

- Move existing detailed content from README into organized docs:
- Configuration reference
- Local development setup (Node.js flow)
- Architecture / how it works
- Advanced options (incremental indexing, watch mode, multi-repo, etc.)
- Troubleshooting / FAQ
- The implementer has latitude to decide how much restructuring vs. new content is needed — the goal is approachability, not completeness

### Scope boundaries

- Focus on Docker as the primary getting-started path
- Keep independent from elastic/semantic-code-search#18 (agentic interfaces CLI) and elastic/semantic-code-search#19 (index naming) — those can be added to the guide later
- Existing content is valuable — restructure and resurface it, don't throw it away

## Acceptance Criteria

- [ ] README.md is a concise getting-started guide focused on the Docker workflow
- [ ] A new developer can go from zero to indexed repo by following the README linearly
- [ ] Dense/detailed content moved to `docs/` with clear linking from README
- [ ] No valuable existing content lost — restructured, not deleted