Phase 1: Adding documentation metadata tags

[evankanderson]

Proposed Changes

In preparation for a CNCF TechDocs Assessment, start labelling Knative documentation with metadata to assist with re-organizing content to different information architecture options (we will still need manual review, but this may simplify that review).

I'm using a schema with 3 fields:

field	allowed values	explanation
audience	developer, administrator, buyer	Who the target audience is; ensure documents have a clear, single audience
components	eventing, functions, serving (list)	What components are featured on the page.
function	marketing, community, tutorial, how-to, reference, explanation	A diataxis tag or "marketing" or "community" indicating the needs addressed by the documentation.

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[evankanderson]

https://diataxis.fr/complex-hierarchies/#two-dimensional-problems may be helpful when figuring out how to structure these eventually.