Add CVSS v4 crosswalk documentation

[ahouseholder]

Inspired by CERTCC/Vultron#6, as well as with a similar goal of integration as #314, we should create a document explaining how SSVC and CVSSv4 intersect/overlap. As in #314, I think this might be as simple as a table like:

CVSSv4 vector element	SSVC decision point(s)	Notes
...	...	...

or reverse the first two columns, or do whatever makes sense.

Depends on

• #284
• #392
• #394

[ahouseholder]

Extends #284

[j---]

This also extends the discussion in https://github.com/CERTCC/SSVC/blob/main/docs/topics/related_systems.md