[ui-doc] Documenting data-testid attribute in contributing-frontend.md file

[ananya-agarwal]

What changes were proposed in this pull request?

This PR is about documenting the data-testid attribute added along with the HTML elements in Hue. The data-testid have been added in the PR: https://github.com/cloudera/hue/pull/3652

How was this patch tested?

This is just a documentation PR in contributing-frontend.md file and needs no testing.

Please review Hue Contributing Guide before opening a pull request.