Cross-reference Requirements to Use Cases

Resolve #18

add ID to all headers so visible in rendering
add links from requirements to ucs
format so links are in vertical column (I tried using a comma-separated list at first; this was more compact but much much harder to read)
remove UC-PW (really a feature and not a use case) and add two other use cases, UC-BR (for web apps) and UC-CM (for IoT devices) where persistence would be useful.

To do (can do in separate PR, will discuss in call)

trim requirements links so a requirement points only at a use cases that needs it. right now, except for one case where we already had a more specific list, all requirements point to all use cases. One issue is that some requirements are not related to use cases but to the implementation, i.e. Scalability. A user cares about performance, scalability is a provider's requirement, and it might also not been needed in some cases, depending on assumptions (a user offloading AR/VR rendering to their own desktop) but the same use case may need it with other assumptions (a user offloading AR/VR rendering to an edge cloud).
Requirements like scalability may relate to a stakeholder rather than a use case. Also, we have a stakeholder table but are not using it. Should we perhaps add a column to the requirements table to relate each requirement to stakeholders? Then scalability may be a requirement of a CSP and CDN but not a HW or ISP stakeholder, for instance.

w3c / edge-computing-web-exploration