docs/runtime: Update attestation documentation

[martintomazic]

Update out-dated runtime host protocol documentation, with a focus on attestation.

Finally, given that Remote Attestation is completely Intel specific it would be nice to expand on:

1. Why we still only support Intel:
   • Rn this is intentional as it gives by far the most security guarantees (be concrete what others lack).
2. Overview of the architectural enclaves (AE) and explain why such architecture is needed.
   • This is critical for understanding attestation flow, attestation values and policy parameters.
   • Should be a separate chapter, e.g. recommended reading prior to reading RHP attestation chapter.
3. Link specific architectural enclaves (e.g. QE, PCE) from 2. to RHP attestation steps.

Motivation:

Continuous TEE attestation together with on-chain policy and verification are of one the main value propositions of our protocol. Hence, we should take special effort at documenting them.

This enables:

• Articles/guides to directly reference deep technical documentation (e.g. https://github.com/oasisprotocol/docs/issues/1524).
• Eases protocol audit for (technical) external partners / users that want to start building (and trust) Oasis.
• Set-up-tee or troubleshooting sections to directly link to the problematic concepts so that operator/auditor can understand what and why.
• On-boarding, reference speakers can use etc.

Plan of attack / ETA

I am hoping to start working on this in Q1/2 after merging some attestation related PRs which should give me a solid understanding of the concepts above.

Prior to that, PRs, ideas how to organize things and what to focus on are welcome!

Also happy to make sub-issues. E.g. Outdated RHP attestation specific stuff could be updated relatively fast.