Add a link to open a new issue into the API docs

[huningxin]

@anssiko mentioned in webmachinelearning/webnn-samples#4 (comment)

aybe we should consider adding a feedback mechanism as well in the API docs (e.g. simply add a link to open a new issue in the webnn-polyfill repo) to capture web developer feedback early. I suspect API docs is much more familiar interface to learn about the API than the web spec repo for web developers, and we might capture more such feedback that way.