doc: crypto: document SPKAC additions

Document the SPKAC functionality that was added in commit 7f66e44.
Revert "crypto: add SPKAC support"

This reverts commit 7f66e44dc1e90e7abda2a9ed02d7e8163e1f6358.
Revert "doc: crypto: document SPKAC additions"

This reverts commit aa94450b31a28d6251df3b88fdd652ddd1e67a69.
crypto: add SPKAC support

Implements new class 'Certificate' within crypto object for working
with SPKAC's (signed public key & challenge) natively.
doc: crypto: document SPKAC additions

Document the SPKAC functionality that was added in commit 7bf46ba.