Docs: Add new code example to stub endpoint return nimn or msgpack data

[amitguptagwl]

Index.html on gh-pages branch need to be updated to add a code example about how to stub an endpoint which is expected to return msgpack or nimn data.

Please refer mapping page for response.parser property which will be used in the example.

Feel free to discuss any doubt regarding this issue.

First time contributors may check how to guide about how to clone a repo, create PR etc.

[amitguptagwl]

wiki: https://github.com/NaturalIntelligence/Stubmatic/wiki/03.3-Parsers