[otbn] Specify recommended software countermeasures

We currently have a list of threats and attacks in our design doc, as well as hardware countermeasures against them. However, this gives an incomplete picture of our countermeasures as long as we don't also list the recommended software countermeasures.

Add such countermeasures to the design doc and map them to attacks.

Related to #3314

Start with the OTBN design document and review its existing threat, attack, and hardware-countermeasure lists. Use related issue #3314 for context on the expected scope. Done means the design document lists the recommended software countermeasures and maps them to the relevant attacks.

Written by the indexing model from the issue text.