Add Panxo bidder adapter documentation (prebid#6419)

Adding documentation for the Panxo bid adapter which was merged
in Prebid.js release 10.23.0 (PR #14351).

Panxo is a specialized SSP for AI-referred traffic monetization.

Features documented:
- TCF/GDPR support (GVL ID: 1527)
- USP/CCPA support
- GPP support
- COPPA support
- Supply Chain (schain) support
- Price Floors support
- First Party Data support
- Deals support

Co-authored-by: Joan Faura <joanfaura@MacBook-Pro.local>

Author: css12s

dev-docs/bidders/panxo.md

@@ -0,0 +1,96 @@
+---
+layout: bidder
+title: Panxo
+description: Prebid Panxo Bidder Adapter
+biddercode: panxo
+tcfeu_supported: true
+gvl_id: 1527
+usp_supported: true
+coppa_supported: true
+gpp_supported: true
+schain_supported: true
+floors_supported: true
+fpd_supported: true
+media_types: banner
+safeframes_ok: true
+bidder_supports_deals: true
+pbjs: true
+pbs: false
+prebid_member: true
+multiformat_supported: will-not-bid
+ortb_blocking_supported: false
+sidebarType: 1
+---
+
+### Before You Begin
+
+The Panxo adapter requires the Panxo Signal script to be installed on your page before Prebid.js loads. Please register at [app.panxo.ai](https://app.panxo.ai) to obtain your property key and Signal script endpoint.
+
+**Important**: Without the Signal script setting the `panxo_uid` in localStorage, the adapter will not participate in the auction.
+
+### Bid Params
+
+{: .table .table-bordered .table-striped }
+| Name | Scope | Description | Example | Type |
+|---------------|----------|--------------------------------------------------|----------------------|----------|
+| `propertyKey` | required | Property identifier from Panxo dashboard | `'abc123def456'` | `string` |
+| `floor` | optional | Minimum CPM floor price in USD | `0.50` | `number` |
+
+### Setup Example
+
+```html
+<!-- Step 1: Panxo Signal Script (MUST load before Prebid) -->
+<script async src="https://cdn.panxo-sys.com/o/YOUR_ENDPOINT_KEY"></script>
+
+<!-- Step 2: Prebid.js -->
+<script async src="prebid.js"></script>
+```
+
+```javascript
+var adUnits = [{
+    code: 'banner-ad',
+    mediaTypes: {
+        banner: {
+            sizes: [[300, 250], [728, 90]]
+        }
+    },
+    bids: [{
+        bidder: 'panxo',
+        params: {
+            propertyKey: 'your-property-key'
+        }
+    }]
+}];
+```
+
+### First Party Data
+
+This adapter supports First Party Data via `ortb2`:
+
+- `ortb2.site.*` (name, cat, content, keywords, etc.)
+- `ortb2.user.*` (data, ext)
+
+### User Sync
+
+Panxo supports pixel-based user sync:
+
+```javascript
+pbjs.setConfig({
+    userSync: {
+        filterSettings: {
+            pixel: {
+                bidders: ['panxo'],
+                filter: 'include'
+            }
+        }
+    }
+});
+```
+
+### Additional Notes
+
+- **Net Revenue**: Bids are returned as net revenue.
+- **TTL**: 300 seconds (5 minutes)
+- **Currency**: USD
+- **Safeframes**: Supported
+- **Advertiser Domains**: Available in bid responses at `meta.advertiserDomains`