[doc] document our primitives

Nobody has claimed this yet.

- [ ] add mention in Comportability the recommendation to follow utilization guides for existing IP primitives
- [ ] pointer to a yet-to-be-written technical specification for all primitives (see below)
- [ ] list requirements for primitives (RTL, lint, DV, etc)
- [ ] suggest the protocol to add a primitive to our library (rationale as to why existing ones are insufficient)
- [ ] create said technical specification for all primitives; goal: concise but thorough, with rationale;
- [ ] Question: one master doc, or one per? (leaning former)
- [ ] Question: place under hw/ip/prim/doc/prim.md? or hw/ip/prim/README.md?
- [ ] Sections, similar to other technical specifications: feature set, overview, block diagram, interfaces, current level of verification (mini D/V status), CDC implications (a bit redundant with hw/formal/README.md section on CDC that is coming), regtool recommendations, software recommendations

Start by reviewing the existing Comportability guidance and the referenced hw/formal/README.md section on CDC. Compare hw/ip/prim/doc/prim.md and hw/ip/prim/README.md as possible locations, then define whether the specification is master or per-primitive and cover the listed requirements, verification, interfaces, CDC, regtool, and software sections. Done means the checklist and open placement questions are resolved in concise primitive documentation.

Written by the indexing model from the issue text.