Create a detailed, low level, design doc for attestation using SGX

[dpal]

The following ticket tracked the high-level design of the attestation flow.
enarx/enarx#2230
The document is here: https://hackmd.io/KHyHqYHFTSukTOvMjW9N2Q

The ticket is closed now, and this ticket is open for creating a low-level detailed end-to-end attestation flow for the specific architecture - SGX.

The acceptance criteria:

• Document describes all the details of the specific data that goes into the attestation, the ins and outs.
• Document describes collecting the necessary data, from where and using which calls.
• Document includes the detailed workflow of every step involved in the attestation focusing on the specific architecture (SGX).
• Document covers all the details and structure of the resulting artifacts (Certificate, CSR)

[rjzak]

Copying from #124:

Should be a markdown document in the SGX attestation crate.