nitro_enclaves: Add overview documentation

nitro_enclaves: Add overview documentation

Add documentation on the overview of Nitro Enclaves. Include it in the
virtualization specific directory.

Changelog

v9 -> v10

* Update commit message to include the changelog before the SoB tag(s).

v8 -> v9

* Move the Nitro Enclaves documentation to the "virt" directory and add
  an entry for it in the corresponding index file.

v7 -> v8

* Add info about the primary / parent VM CID value.
* Update reference link for huge pages.
* Add reference link for the x86 boot protocol.
* Add license mention and update doc title / chapter formatting.

v6 -> v7

* No changes.

v5 -> v6

* No changes.

v4 -> v5

* No changes.

v3 -> v4

* Update doc type from .txt to .rst.
* Update documentation based on the changes from v4.

v2 -> v3

* No changes.

v1 -> v2

* New in v2.

Reviewed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
Link: https://lore.kernel.org/r/20200921121732.44291-18-andraprs@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>