docs(admin): add Data-drive encryption (LUKS) article

Covers the opt-in LUKS2 encryption of /docker available on new server
installs. Reboot UX (web unlock at :8444, SSH fallback), threat model
in plain English, what LUKS does and doesn't protect against, header
backup handling, passphrase rotation outline, and the "do not enable
this if you need unattended reboots" caveat.

Sidebar order 7 (after Backups in the admin section).
Linked from the admin overview "What's in this section" list and from
the VDS comparison section of "What is containerized hosting?".

No cost / pricing language by design — operational positioning is
still being decided.

src/content/docs/whp/admin/overview.mdx

@@ -52,6 +52,7 @@ When you sign in as a super admin, these sections appear in addition to the cust
 - **[AI Monitor, Issues & Ignore Rules](/whp/admin/site-monitoring/)** — the three pages that drive the Site Monitoring add-on.
 - **[Users & delegated access](/whp/admin/user-management/)** — create accounts, set account types, delegate access, and handle suspensions.
 - **[Backups](/whp/admin/backups/)** — configure the default backup target so customer auto-backups start running. Full-server backups are a separate, plan-dependent concern.
+- **[Data-drive encryption (LUKS)](/whp/admin/data-drive-encryption/)** — optional at-rest encryption of the `/docker` data volume. Available only on new server installs; adds a manual unlock step after every reboot.
 
 ## Things to know before you change server-wide settings