docs: refactor guides for clearer navigation (#795)

## ℹ️ Description
Refactors and reorganizes documentation to improve navigation and keep
the README concise.

- Link to the related issue(s): Issue #N/A
- Describe the motivation and context for this change.
- The README had grown long and duplicated detailed config/ad
references; this consolidates docs into focused guides and adds an
index.

## 📋 Changes Summary
- Add dedicated docs pages for configuration, ad configuration, update
checks, and a docs index.
- Slim README and CONTRIBUTING to reference dedicated guides and clean
up formatting/markdownlint issues.
- Refresh browser troubleshooting and update-check guidance; keep the
update channel name aligned with schema/implementation.
- Add markdownlint configuration for consistent docs formatting.

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [ ] 🐞 Bug fix (non-breaking change which fixes an issue)
- [x] ✨ New feature (adds new functionality without breaking existing
usage)
- [ ] 💥 Breaking change (changes that might break existing user setups,
scripts, or configurations)


## ✅ Checklist
Before requesting a review, confirm the following:
- [x] I have reviewed my changes to ensure they meet the project's
standards.
- [x] I have tested my changes and ensured that all tests pass (`pdm run
test`).
- [x] I have formatted the code (`pdm run format`).
- [x] I have verified that linting passes (`pdm run lint`).
- [x] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Reorganized and enhanced contributing guidelines with improved
structure and formatting
* Streamlined README with better organization and updated installation
instructions
* Added comprehensive configuration reference documentation for
configuration and ad settings
* Improved browser troubleshooting guide with updated guidance,
examples, and diagnostic information
  * Created new documentation index for easier navigation

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

docs/INDEX.md

@@ -0,0 +1,33 @@
+# Documentation Index
+
+This directory contains detailed documentation for kleinanzeigen-bot users and contributors.
+
+## User Documentation
+
+- [Configuration](./CONFIGURATION.md) - Complete reference for `config.yaml`, including all configuration options, timeouts, browser settings, and update check configuration.
+
+- [Ad Configuration](./AD_CONFIGURATION.md) - Complete reference for ad YAML files, including automatic price reduction, description prefix/suffix, and shipping options.
+
+- [Browser Troubleshooting](./BROWSER_TROUBLESHOOTING.md) - Troubleshooting guide for browser connection issues, including Chrome 136+ security requirements, remote debugging setup, and common solutions.
+
+## Contributor Documentation
+
+Contributor documentation is located in the main repository:
+
+- [CONTRIBUTING.md](../CONTRIBUTING.md) - Development setup, workflow, code quality standards, testing requirements, and contribution guidelines.
+
+- [TESTING.md](./TESTING.md) - Detailed testing strategy, test types (unit/integration/smoke), and execution instructions for contributors.
+
+## Getting Started
+
+New users should start with the [README](../README.md), then refer to these documents for detailed configuration and troubleshooting information.
+
+### Quick Start (3 steps)
+
+1. Install and run the app from the [README](../README.md).
+2. Generate `config.yaml` with `kleinanzeigen-bot create-config` and review defaults in [Configuration](./CONFIGURATION.md).
+3. Verify your setup with `kleinanzeigen-bot verify`, then publish with `kleinanzeigen-bot publish`.
+
+### Common Troubleshooting Tips
+
+- Browser connection issues: confirm remote debugging settings and Chrome 136+ requirements in [Browser Troubleshooting](./BROWSER_TROUBLESHOOTING.md).