Change `dictionary_compression.c` example to use API for dictionary creation

**Is your feature request related to a problem? Please describe.**
The current example in [`examples/dictionary_compression.c`](https://github.com/facebook/zstd/blob/dev/examples/dictionary_compression.c) assumes that the user has already created the dictionary via the CLI. I checked this example specifically to figure out how to do dictionary compression from end-to-end with the API, so in my case this fails as an example considering it's missing the most important step in the process.

**Describe the solution you'd like**
Adding a function to demonstrate the usage of the API for dictionary creation would make the example infinitely more useful.

**Describe alternatives you've considered**
I suppose that an entirely new example, something like `dictionary_creation.c`, would also solve this issue.

**Additional context**
n/a