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>

astro.config.mjs

@@ -20,8 +20,8 @@ export default defineConfig({
     '/whp/add-ons/': '/whp/add-ons/overview/',
     '/whp/admin/': '/whp/admin/overview/',
     '/domains/': '/domains/transferring-a-domain-to-us/',
-    '/email/': '/email/outbound-spam-filtering/',
     '/support/': '/support/remote-support/',
+    '/cpanel/': '/cpanel/nameservers/',
   },
   vite: {
     resolve: {
@@ -89,12 +89,12 @@ export default defineConfig({
           ],
         },
         {
-          label: 'Domains',
-          items: [{ autogenerate: { directory: 'domains' } }],
+          label: 'cPanel hosting',
+          items: [{ autogenerate: { directory: 'cpanel' } }],
         },
         {
-          label: 'Email',
-          items: [{ autogenerate: { directory: 'email' } }],
+          label: 'Domains',
+          items: [{ autogenerate: { directory: 'domains' } }],
         },
         {
           label: 'Support',