spectest index command standard outputs the index markdown file path after generated it.

[nao1215]

Is your feature request related to a problem? Please describe.

After generating the index markdown, spectest index does not output anything. I think it is difficult for the user to understand what happened with this specification. Therefore, the generated index file path will be outputted to the standard output.

Describe the solution you'd like

If markdown.GenerateIndex() completes successfully, it will output the index file path to the standard output. If an error occurs, it will directly return the error to the higher-level function.
https://github.com/go-spectest/spectest/blob/26de85c47a740f596035376cf4b84e09ac9c7fc2/cmd/spectest/sub/index.go#L59-L62

[nao1215]

Done #210