docs(developer): a page for the browser variables, and links from where they are used

browser_info and browser_url_vars were only described inside the troubleshooting
page, under Support Diagnostics. That is the right place to find them if you
already know they exist and are looking for a log line; it is not where anyone
looks when writing a hook, which is what they are mostly for.

The new Developer Guide page carries the whole subject: which services receive
the two tables, the column lists, the debug values per platform and the MPEINIT
log lines, reading them from a hook with the exist() guard, and the reminder that
they are client-supplied and so diagnostics rather than a security boundary.

Troubleshooting keeps three stub headings - browser_info, browser_url_vars,
reading-them-from-a-hook - pointing at the page, so the anchors that are already
published keep resolving, and the detail is not maintained twice.

Linked from the three places a hook author actually starts: the HOOK Scripts
section of the MPE_TABLES page, the pre-edit hook guide (where the editor's
inputs are listed), the two-rule-sets guide (post-edit hook), and the VA embed
page, where embed=va is exactly the kind of thing a hook wants to branch on.

docs/embed-va.md

@@ -53,6 +53,8 @@ In the VA Report Designer, add a **Data-Driven Content** object and set the URL
 
 ## Debugging
 
+A hook script can tell an embedded editor from a standalone one without any client-side inspection: `work.browser_info` carries the page URL and `referrer`, and `work.browser_url_vars` carries `embed=va`. See [Browser Variables](/developer-guide/browser-variables/).
+
 The following snippet can be used in console to dump the values being provided to DC from VA:
 
 ```