improve documentation for interacting with Pages API

[prayankmathur]

After having a discussion with @shiftkey and referring to the docs, we found that any normal integration tests won't be of any use as you need to be an authenticated owner of the repository to use the Pages API.

Refer to #1213 and especially conversation after this comment for more details.

[shiftkey]

This is mentioned in the docs: https://developer.github.com/v3/repos/pages/

Information about the site and the builds can only be accessed by authenticated owners, even though the websites are public.

[github-actions]

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