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])
