feat(python): Document default span attrs in Tornado, update for 3.0 (#14366)

Document what span attributes will be attached to transactions by
default by our builtin integrations. This PR updates Tornado. Rest will
follow.

I created a new 3.x page for this since it's new behavior in 3.0.

Author: sentrivana

docs/platforms/python/integrations/tornado/index__v3.x.mdx

@@ -0,0 +1,85 @@
+---
+title: Tornado
+description: "Learn about using Sentry with Tornado."
+---
+
+The Tornado integration adds support for the [Tornado web framework](https://www.tornadoweb.org/).
+
+## Install
+
+Install `sentry-sdk` from PyPI with the `tornado` extra:
+
+```bash {tabTitle:pip}
+pip install "sentry-sdk[tornado]"
+```
+```bash {tabTitle:uv}
+uv add "sentry-sdk[tornado]"
+```
+
+## Configure
+
+If you have the `tornado` package in your dependencies, the Tornado integration will be enabled automatically when you initialize the Sentry SDK.
+
+<PlatformContent includePath="getting-started-config" />
+
+## Verify
+
+```python
+import asyncio
+import tornado
+
+sentry_sdk.init(...)  # same as above
+
+class MainHandler(tornado.web.RequestHandler):
+    def get(self):
+        1 / 0  # raises an error
+        self.write("Hello, world")
+
+def make_app():
+    return tornado.web.Application([
+        (r"/", MainHandler),
+    ])
+
+async def main():
+    app = make_app()
+    app.listen(8888)
+    await asyncio.Event().wait()
+
+asyncio.run(main())
+```
+
+When you point your browser to [http://localhost:8888/](http://localhost:8888/) a transaction in the Performance section of [sentry.io](https://sentry.io) will be created. Additionally, an error event will be sent to [sentry.io](https://sentry.io) and will be connected to the transaction.
+
+It takes a couple of moments for the data to appear in [sentry.io](https://sentry.io).
+
+## Behavior
+
+- The Tornado integration will be installed for all of your apps and handlers.
+- All exceptions leading to a Internal Server Error are reported.
+- Request data is attached to all events: **HTTP method, URL, headers, form data, JSON payloads**. Sentry excludes raw bodies and multipart file uploads. Sentry also excludes personally identifiable information (such as user ids, usernames, cookies, authorization headers, IP addresses) unless you set `send_default_pii` to `True`.
+- Each request has a separate scope. Changes to the scope within a view, for example setting a tag, will only apply to events sent as part of the request being handled.
+- Logging with any logger will create breadcrumbs when the [Logging](/platforms/python/integrations/logging/) integration is enabled (done by default).
+
+### Tracing
+
+A set of predefined span attributes will be attached to Tornado transactions by default. These can also be used for sampling since they will also be accessible via the `sampling_context` dictionary in the [`traces_sampler`](/platforms/python/configuration/options/#traces_sampler).
+
+- `url.path`
+- `url.query`
+- `url.scheme`
+- `url.full`
+- `http.request.method`
+- `http.request.header.{header}`
+- `server.address`
+- `server.port`
+- `network.protocol.name`
+- `network.protocol.version`
+
+These attributes will also be sent to Sentry. If you don't want that, you can filter them out using a custom [`before_send`](/platforms/python/configuration/options/#before_send) function.
+
+## Supported Versions
+
+- Tornado: 6+
+- Python: 3.8+
+
+<Include name="python-use-older-sdk-for-legacy-support.mdx" />