x86/cpu: Add "Old Microcode" docs to hw-vuln toctree

x86/cpu: Add "Old Microcode" docs to hw-vuln toctree

Sphinx reports missing toctree entry warning:

Documentation/admin-guide/hw-vuln/old_microcode.rst: WARNING: document isn't included in any toctree

Add entry for "Old Microcode" docs to fix the warning.

Fixes: 4e2c719782a847 ("x86/cpu: Help users notice when running old Intel microcode")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20250502023358.14846-1-bagasdotme%40gmail.com