create a document about design details of mesatee-sgx

I'm recently reading the source code of `sgx_tstd`,`sgx_types`,`sgx_libc`,and so on.
It's quite interesting and useful for users and developers to know about the design details of `mesatee-sgx`.

I would like to create a documents about architecture of source code, but it may be weeks ago.
By the way, I'm not at english writing but I'll try my best to do it and the documents may need to be reviewed. :smile:

Start by reading the source of sgx_tstd, sgx_types, and sgx_libc to understand how the SDK components fit together. Document the source architecture and design details, then seek review of the resulting documentation, especially for English clarity and technical accuracy.

Written by the indexing model from the issue text.