sgx.7: New page with overview of Software Guard eXtensions (SGX)

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

v11:
* Address Reinette's remarks for v10:
  https://lore.kernel.org/linux-sgx/8f84b8e8-b478-bb81-4aa8-536df882a144@intel.com/
v10:
* v9 was malformed, essentially a resend.
v9:
* Rename "Address Space" section as "Memory mapping", and refine the
  text.
* Rename "Ioctls" section as "Construction", and refine the text.
v8:
* Fix errors reported for the previous version.
v7:
* Added more meat about the address space and API.
* Reorganized the text to have focus more on developer to have a big
  picture of kernel provided interfaces.
v6:
* Small fixes based on Dave's and Reinette's feedback.
* Extended the "Permissions" section to cover mmap()
v5:
* Taking away hardware concepts and focusing more on the interface.
v4:
* Did a heavy edit trying to streamline the story a bit and focus on
  stuff important to the user (e.g. lighten up x86 details).
v3:
* Overhaul based on Michael's comments. Most likely needs to be refined
  in various places but this is at least a small step forward for sure.
v2:
* Fixed the semantic newlines convention and various style errors etc.
  that were reported by Alenjandro and Michael.
* SGX was merged to v5.
2 files changed