Documentation/x86: Add AMD Hardware Feedback Interface documentation

Introduce a new documentation file, `amd_hfi.rst`, which delves into the
implementation details of the AMD Hardware Feedback Interface and its
associated driver, `amd_hfi`. This documentation describes how the driver
provides hint to the OS scheduling which depends on the capability of core
performance and efficiency ranking data.

This documentation describes:

  * The design of the driver
  * How the driver provides hints to the OS scheduling
  * How the driver interfaces with the kernel for efficiency ranking data.

Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/20250609200518.3616080-2-superm1@kernel.org