docs: Improve README clarity and fix configuration documentation (#711)

## ℹ️ Description

This PR addresses issue #708 by improving the README's About section to
make the bot's purpose clearer to new users. It also fixes a technical
inaccuracy in the configuration documentation.

- Link to the related issue(s): Issue #708
- **Motivation**: The current About section uses ambiguous terminology
("ads" instead of "listings") and doesn't clearly communicate what the
bot does. Additionally, the configuration example incorrectly documents
`shipping_costs` as available in `ad_defaults`, when it's only
implemented for per-ad configuration.

## 📋 Changes Summary

**About Section Improvements:**
- Changed "ads" to "listings" for clarity (addresses confusion mentioned
in #708)
- Added "Key Features" section with 6 concrete capabilities
- Added "Why This Project?" section explaining the rewrite and
differences from legacy client
- Preserved all legal disclaimers

**Configuration Documentation Fix:**
- Removed `shipping_costs` from `ad_defaults` example (not implemented
in `AdDefaults` Pydantic class)
- Added clarifying comment that `shipping_costs` and `shipping_options`
must be configured per-ad
- Verified `shipping_costs` remains documented in ad configuration
section

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

*Note: This is a documentation-only change with no code modifications.*

## ✅ Checklist
- [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 (N/A -
documentation only).
- [x] I have formatted the code (N/A - documentation only).
- [x] I have verified that linting passes (N/A - documentation only).
- [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.

README.md

@@ -24,8 +24,15 @@ For details on the new smoke test strategy and contributor guidance, see [TESTIN
 
 ## <a name="about"></a>About
 
-**kleinanzeigen-bot** is a console-based application to simplify the process of publishing ads on kleinanzeigen.de.
+**kleinanzeigen-bot** is a command-line application to **publish, update, delete, and republish listings** on kleinanzeigen.de.
-It is a spiritual successor to [Second-Hand-Friends/ebayKleinanzeigen](https://github.com/Second-Hand-Friends/ebayKleinanzeigen).
+
+### Key Features
+- **Automated Publishing**: Publish new listings from YAML/JSON configuration files
+- **Smart Republishing**: Automatically republish listings at configurable intervals to keep them at the top of search results
+- **Bulk Management**: Update or delete multiple listings at once
+- **Download Listings**: Download existing listings from your profile to local configuration files
+- **Browser Automation**: Uses Chromium-based browsers (Chrome, Edge, Chromium) for reliable automation
+- **Flexible Configuration**: Configure defaults once, override per listing as needed
 
 ### ⚠️ Legal Disclaimer
 
@@ -261,7 +268,7 @@ ad_defaults:
 
   price_type: NEGOTIABLE # one of: FIXED, NEGOTIABLE, GIVE_AWAY, NOT_APPLICABLE
   shipping_type: SHIPPING # one of: PICKUP, SHIPPING, NOT_APPLICABLE
-  shipping_costs: # e.g. 2.95
+  # NOTE: shipping_costs and shipping_options must be configured per-ad, not as defaults
   sell_directly: false # requires shipping_options to take effect
   contact:
     name: ""