One set of mail variables for every mail, and {link} in state mails

{site} and {site_name} everywhere; a state mail adds {self_link} (the
submitter's own page, when their record's state allows a move there),
{desk_link} and {link}, which picks by who the mail is to. gullberg's
{link} mails went out blank until now. Documented in docs/mail.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MznPTC31w5mSUK4XMehcpi

docs/SUMMARY.md

@@ -7,6 +7,7 @@
 - [Getting started](getting-started.md)
 - [Content reference](content-reference.md)
 - [State machines](state-machines.md)
+- [Mail](mail.md)
 - [Access](access.md)
 - [Storylines](storylines.md)
 - [Memberships](memberships.md)