feat(perf): Add span data conventions

Author: AbhiPrasad

src/docs/sdk/event-payloads/span.mdx

@@ -32,6 +32,8 @@ import "./properties/tags.mdx";
 
 : _Optional_. Arbitrary data associated with this Span.
 
+The semantic conventions for the `data` field are described in <Link to="/sdk/performance/span-data-conventions/">Sentry's Span Convention Documentation</Link>.
+
 ```json
 {
   "data": {

src/docs/sdk/performance/span-data-conventions.mdx

@@ -0,0 +1,30 @@
+---
+title: 'Span Data Conventions'
+---
+
+The Span Interface specifies a series of _timed_ application events that have a start and end time. Below describes the conventions for the Span interface for the `data` field on the span.
+
+The `data` field on the span is expected to follow [OpenTelemetry's semantic conventions for attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/24de67b3827a4e3ab2515cd8ab62d5bcf837c586/specification/trace/semantic_conventions/README.md) as much as possible.
+
+Keys on the `data` field should be lower-case and use underscores instead of camel-case. There are some exceptions to this, but these exist because of backwards compatability.
+
+Below describes the conventions for the Span interface for the `data` field on the span and how they are currently used by the product.
+
+## Mobile
+
+| Attribute                                                                                             | Type              | Description                                                                                          | Examples |
+| ----------------------------------------------------------------------------------------------------- | ----------------- | ---------------------------------------------------------------------------------------------------- | -------- |
+| `blocked_main_thread`                                                                                 | boolean           | Whether the main thread was blocked by the span.                                                     | `true`   |
+| `call_stack`                                                                                          | Array<StackFrame> | The most relevant stack frames, that lead to the File I/O span. The stack frame should adhere to the |
+| [`StackFrame`](https://develop.sentry.dev/sdk/event-payloads/stacktrace/#frame-attributes) interface. |                   |
+
+## Browser
+
+| Attribute           | Type   | Description                                | Examples              |
+| ------------------- | ------ | ------------------------------------------ | --------------------- |
+| `url`               | string | The URL of the resource that was fetched.  | `https://example.com` |
+| `method`            | string | The HTTP method used.                      | `GET`                 |
+| `type`              | string | The type of the resource that was fetched. | `xhr`                 |
+| `Encoded Body Size` | string | The encoded body size of the request.      | `123 B`               |
+| `Decoded Body Size` | string | The decoded body size of the request.      | `456 B`               |
+| `Transfer Size`     | string | The transfer size of the request.          | `789 B`               |