New Adapter: Start.io (prebid#6018)

* Add initial documentation for Start.io adapters

* Adjustments based on review comments

Author: prebid-startio

dev-docs/bidders/startio.md

@@ -0,0 +1,69 @@
+---
+layout: bidder
+title: Start.io
+biddercode: startio
+description: Prebid Start.io Adapter
+tcfeu_supported: true
+coppa_supported: false
+gpp_supported: false
+floors_supported: false
+media_types: banner, video, native
+multiformat_supported: will-bid-on-any
+safeframes_ok: false
+schain_supported: false
+gvl_id: 1216
+usp_supported: false
+pbjs: true
+pbs: true
+prebid_member: false
+fpd_supported: false
+privacy_sandbox: no
+ortb_blocking_supported: false
+sidebarType: 1
+---
+
+### Before You Begin
+
+The Start.io bidder adapter requires additional setup and approval from the Start.io team. For additional information, please reach out to <prebid@start.io>.
+
+### Bid Params
+
+{: .table .table-bordered .table-striped }
+| Name          | Scope    | Description                       | Example                | Type         |
+|---------------|----------|-----------------------------------|------------------------|--------------|
+| `publisherId` | required | Publisher ID                      | `'publisher-1234'`     | `string`     |
+
+#### Configuration
+
+Sample banner setup:
+
+```js
+var adUnits = [
+  {
+    code: "div-gpt-ad-12345-0",
+    mediaTypes: {
+      banner: {
+        sizes: [[300, 250]]
+      }
+    },
+    bids: [
+      {
+        bidder: "startio",
+        params: {
+          publisherId: "publisher-12345"
+        }
+      }
+    ]
+  }
+]
+
+pbjs.que.push(function() {
+  pbjs.addAdUnits(adUnits);
+});
+```
+
+### Additional Notes
+
+#### Request and Response Attributes
+
+- Bids are returned in **net** - that is, the bids returned reflect the bid amount with revenue sharing already taken into account. No adjustment is necessary.