gguf.md: Add GGUF Naming Convention Section (#822)

gguf.md: Add GGUF Naming Convention Section (#822)

* gguf.md: Add GGUF Naming Convention Section

* gguf.md: add BF16

* gguf.md: GGUF Filename Parsing Strategy

* gguf.md: include tensor type table and historical context

* gguf.md: minor corrections

* gguf.md: more detailed breakdown of tensor type mapping

* gguf.md: use Encoding Scheme name instead

* gguf.md: minor correction to overall naming convention

* gguf.md: simplify GGUF Naming Convention