docs: add Email section — client setup + Baruwa retirement notice

Replaces the three Outlook 2016 articles (WHMCS 2, 3, 9 — ~62k views)
with one client-agnostic setup guide. Outlook 2016 went end of life in
October 2025, and the two archiving articles documented a feature the
current Outlook doesn't have.

Server settings are taken from the live autoconfig endpoint rather than
the old articles: IMAP 993/SSL, POP3 995/SSL, SMTP 587/STARTTLS, username
is the full address. cPanel settings verified against cpanel01.

Adds a Baruwa retirement notice covering the move to Proxmox Mail Gateway
in November 2026. Deliberately frames this as a platform change rather
than describing our current outbound filtering posture — see the PR
discussion.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/pages/index.astro

@@ -23,10 +23,10 @@ const PRODUCT_META: Record<string, { title: string; blurb: string; firstSection:
     blurb: 'Tips and tricks for getting the most out of WordPress on WHP.',
     firstSection: 'index',
   },
-  'email-clients': {
-    title: 'Email clients',
-    blurb: 'Configure Outlook, Apple Mail, Thunderbird, and mobile clients.',
-    firstSection: 'index',
+  email: {
+    title: 'Email',
+    blurb: 'Set up Outlook, Apple Mail, Thunderbird, and mobile clients — plus news about our filtering.',
+    firstSection: 'set-up-your-email-client',
   },
   domains: {
     title: 'Domains',