docs: add cPanel section; drop the outbound-filtering page

Adds a cPanel hosting section so cPanel customers have a correct
destination — previously this KB was WHP-only, so cPanel-specific
articles had nowhere to redirect to:

- cpanel/nameservers                        (WHMCS 5, 24k views)
- cpanel/free-ssl-certificate               (WHMCS 8, 18k views)
- cpanel/fix-a-403-error                    (WHMCS 6, 20k views)
- cpanel/wordpress-email-from-your-domain   (WHMCS 7, 25k views)

Rewritten rather than copied where the originals were wrong or thin:
- WordPress SMTP told customers to use port 25 unencrypted; now 587/TLS
  with 465 as the implicit-TLS alternative.
- The 403 article now says explicitly that loosening permissions to 777
  makes suEXEC refuse harder, since that's the instinctive wrong fix.
- AutoSSL gained the actual failure modes (CAA records, .htaccess
  redirects intercepting validation, domain not added to the account).

Also removes email/outbound-spam-filtering, added in the previous commit.
Outbound filtering via Baruwa has been disabled and Baruwa is being retired,
so the page describes a feature we no longer offer. It needs rewriting
around the Proxmox migration before it can ship.

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

src/pages/index.astro

@@ -33,10 +33,10 @@ const PRODUCT_META: Record<string, { title: string; blurb: string; firstSection:
     blurb: 'Transfer a domain to us, point it at your hosting, and sort out DNS problems.',
     firstSection: 'transferring-a-domain-to-us',
   },
-  email: {
-    title: 'Email',
-    blurb: 'How our mail filtering works and what to do when a message does not arrive.',
-    firstSection: 'outbound-spam-filtering',
+  cpanel: {
+    title: 'cPanel hosting',
+    blurb: 'Nameservers, SSL certificates, permissions, and email for our cPanel shared and reseller plans.',
+    firstSection: 'nameservers',
   },
   support: {
     title: 'Support',