Add expected User-Agent format to SDK docs (#1222)

* adds user-agent format

* adds info about semicolon separator

* adds examples for basic user agent

* Update src/docs/sdk/overview.mdx

Co-authored-by: Joris Bayer <joris.bayer@sentry.io>

---------

Co-authored-by: Joris Bayer <joris.bayer@sentry.io>

Author: kahest

src/docs/sdk/overview.mdx

@@ -215,6 +215,7 @@ We recommend always sending the following headers:
 
 - `content-type`
 - `content-length`
+- `user-agent`
 
 The following additional headers are permitted as per CORS policy:
 
@@ -229,6 +230,30 @@ The following additional headers are permitted as per CORS policy:
 - `content-encoding`
 - `transfer-encoding`
 
+### User Agent
+
+All SDKs are expected to report their name and version via the `user-agent` header. 
+The following format should be used (unless required or recommended differently by the platform, 
+e.g. for browser SDKs, where the user agent header must be set by the browser itself):
+
+`{sdk-name}/{sdk-version}`
+
+For example:
+- `sentry.python/1.45.0`
+- `sentry.php/4.7.0`
+- `sentry-ruby/5.17.3`
+- `sentry.cocoa/8.24.0`
+
+Additional information about the runtime, operating system, and others can be 
+appended as comments in parentheses, separated by `; ` (semicolon and space), like so:
+
+`{sdk-name}/{sdk-version} ({runtime-name} {runtime-version}; {os-name} {os-version})`
+
+There is no expectation towards the presence or order of fields. The user agent 
+must not contain PII or otherwise sensitive data. In general it should not contain 
+any information that is not already present in the payload.
+
+
 ## Request Compression
 
 SDKs are heavily encouraged to compress the request body before sending it to