v2.1: _DSD Implementation Guide update

Minor updates to the _DSD Implementation Guide.

• Clarify and illustrate use of vendor-defined data in ACPI graph link description. (Issue [#20], PR [#23])
• Add Buffer Data Extension UUID. (Issue [#17], PRs [#18] and [#19])
• Remove invalid restrictive text of the source of UUIDs. (PR [#16])
• Reduce output to only PDF (Issue [#21], PR [#22])

v2.0: Update _DSD Implementation Guide

Update to the new process and format for the _DSD Implementation Guide.

This update formalizes how one is to request and/or specify _DSD device properties. Older usages are deprecated, and a prefix scheme is introduced to allow greater flexibility in the long term.

This is the first release of the guide under these new auspices, and it supersedes any previous versions.