[CDX2.0] guide on perspectives

[jkowalleck]

as perspectives will come for CycloneDX 2.0, we need a write a guide.

The following comments are notes and ideas

[jkowalleck]

we need to mention a certain well-knonw wording/name:

• [FEATURE]: Add support for perspectives specification#736 (comment)

reminder from a Ecma meeting:
there appears to be a term "constituencies" for this thing.
we decided to have this very term in our authoritative guides.