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.

mkdocs.yml

@@ -17,6 +17,7 @@ nav:
 - Developer Guide:
     - Filter in a Pre-Edit Hook: developer-guide/filter-pre-edit-hook.md
     - One Table, Two Rule Sets: developer-guide/two-rule-sets-one-table.md
+    - Browser Variables: developer-guide/browser-variables.md
 - Table Guide:
     - MPE_AUDIT: tables/mpe_audit.md
     - MPE_COLUMN_LEVEL_SECURITY: tables/mpe_column_level_security.md