docs(admin): a page for the message-template editor

The operator half of engagement Phase 5b, which 6.0b of ENGAGEMENT.md assigns to
this repo: what the Templates screen is for, how a shipped default is edited in
place without an upgrade taking the edit back, why variables are clicked rather
than typed, the two halves of every message, draft vs published, the preview and
its dark-mode approximation, test sends, duplicating to make a new template, and
the send log.

Written for someone running a site, not someone reading the design document: it
explains what to do and why the refusals exist, and names no version numbers - the
facts check reads authority from each repo's `main`, and none of this is there yet.

Co-Authored-By: Claude <noreply@anthropic.com>

src/config/sidebar.mjs

@@ -38,6 +38,7 @@ export const docsSidebar = [
       { label: 'Teams', slug: 'docs/administration/teams' },
       { label: 'Moderation', slug: 'docs/administration/moderation' },
       { label: 'Notifications and email', slug: 'docs/administration/notifications-and-email' },
+      { label: 'Message templates', slug: 'docs/administration/message-templates' },
       { label: 'Managing modules', slug: 'docs/administration/managing-modules' },
       { label: 'The shard connection', slug: 'docs/administration/the-shard-connection' },
       { label: 'Maintenance and upgrades', slug: 'docs/administration/maintenance-and-upgrades' },
@@ -113,6 +114,7 @@ export const plannedSidebar = {
     'Teams',
     'Moderation',
     'Notifications and email',
+    'Message templates',
     'Managing modules',
     'The shard connection',
     'Maintenance and upgrades',