feat: document SMS ctx.request_headers

[hperl]

Support for forwarding user request headers to HTTP channel webhooks

A number of common HTTP request headers (see below) are now forwarded to the
webhook when delivering one-time codes via SMS or email. These headers are
available in the ctx.request_headers field in the Jsonnet templates for the
webhook request body.

The list of forwarded headers is the same as for the webhooks in Ory Actions:

Accept
Accept-Encoding
Accept-Language
Content-Length
Content-Type
Origin
Priority
Referer
Sec-Ch-Ua
Sec-Ch-Ua-Mobile
Sec-Ch-Ua-Platform
Sec-Fetch-Dest
Sec-Fetch-Mode
Sec-Fetch-Site
Sec-Fetch-User
True-Client-Ip
User-Agent
X-Forwarded-Host
Ory-Base-Url-Rewrite
Ory-Base-Url-Rewrite-Token
X-Ory-Original-Host
Ory-No-Custom-Domain-Redirect
Cf-Ipcountry