Add support for generating raw dictionaries (or update cli docs to be clear about format)

**Is your feature request related to a problem? Please describe.**
A few people I have worked with have run into issues trying to implement `dcz` content encoding (HTTP compression dictionary transport) because they use the zstd command-line tool to generate the dictionary but `dcz` requires raw dictionary files. Most recently, [here](https://issues.chromium.org/u/1/issues/419147084) on the Chromium issue tracker.

**Describe the solution you'd like**
Optimally, the cli would be updated to allow for generating raw-format dictionary files. If that's not practical, updating the docs and cli help output to be clear that the dictionaries are zstd-specific would probably help.

**Describe alternatives you've considered**
The only other dictionary generator that I'm aware of is the one included in the brotli project's research tools ([here](https://github.com/google/brotli/blob/master/research/dictionary_generator.cc). It works great with zstd as well but requires building from source.

**Additional context**
N/A