Provide definition of "builder" or potentially replace with alternative term

[bobcatfish]

The SLSA requirements refer in a few places to the term builder, however that phrase is not (yet) defined in the terminology.

It would be helpful to have this term included in the terminology; however it might be even better (if it makes sense) to replace it with other terms which are less ambiguous such as platform. builder is especially confusing when the builder is expected to be doing more than just building the artifact, such as fetching source code.

#306 is already requesting that terms like platform and service are used consistently, maybe it makes sense to use the same language instead of builder?

(More context available in SLSA + Tekton: Case Study - the doc is visible to anyone in mailing list [email protected] )

[MarkLodato]

For the text, let's fold this into #306.

For the schema, I don't think it's high enough priority to warrant the breaking change.