doc: add security documentation to provide some background info (#7028)
doc: add security documentation to provide some background info (#7028)

This commit adds some documentation about the design of the
SSE-C and SSE-S3 implementation. It describes how the Minio server
encrypt objects and manages keys.
fwessels/minio,aead/minio,aead/minio,NitishT/minio,minio/minio,donatello/minio,minio/minio,harshavardhana/minio,vadmeste/minio,aead/minio,donatello/minio,harshavardhana/minio,harshavardhana/minio,NitishT/minio,aead/minio,harshavardhana/minio,krishnasrinivas/minio,NitishT/minio,jimjag/minio,balamurugana/minio,jimjag/minio,aead/minio,fwessels/minio,harshavardhana/minio,harshavardhana/minio,jimjag/minio,krishnasrinivas/minio,aead/minio,krisis/minio,NitishT/minio,jimjag/minio,aead/minio,krisis/minio,krishnasrinivas/minio,vadmeste/minio,aead/minio,harshavardhana/minio,balamurugana/minio,fwessels/minio,krishnasrinivas/minio,fwessels/minio,harshavardhana/minio