gguf.md: Add GGUF Naming Convention Section

[mofosyne]

https://github.com/ggerganov/ggml/issues/820

This PR is based on outfile default name generation in https://github.com/ggerganov/llama.cpp/pull/4858, copied from there but removed historical references and justification to why it was designed that way.

Feedback and adjustment will be appreciated. Any changes to this will mean we also need to update llama.cpp default name generation as well.

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In addition, is there any filename generation in this repo? If so then we may want to also update it as well to use this common naming scheme.