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/TESTING.md

@@ -39,14 +39,15 @@ This project uses a layered testing approach, with a focus on reliability and fa
 - All smoke tests **must** be marked with `@pytest.mark.smoke`.
 - Place smoke tests in `tests/smoke/` for discoverability.
 - Example:
-  ```python
-  import pytest
 
-  @pytest.mark.smoke
-  @pytest.mark.asyncio
-  async def test_bot_starts(smoke_bot):
-      ...
-  ```
+```python
+import pytest
+
+@pytest.mark.smoke
+@pytest.mark.asyncio
+async def test_bot_starts(smoke_bot):
+    ...
+```
 
 ### Running Smoke, Unit, and Integration Tests