Doc handling improvements

[Pchelolo]

We need some improvements for docs handling. After this patch, here's how the docs would work:
In the browser:

• https://en.wikipedia.org/api/rest_v1/ would show main swagger page
• https://en.wikipedia.org/api/rest_v1/page/ would give a listing of page-related endpoints in JSON
• https://en.wikipedia.org/api/rest_v1/?doc will 301 redirect to https://en.wikipedia.org/api/rest_v1/
  In curl:
• https://en.wikipedia.org/api/rest_v1/ would give a listing in JSON

As you can see, we don't need the confusing ?doc parameter any more since now it varies on the accept header.

Bug: https://phabricator.wikimedia.org/T141402
cc @wikimedia/services

[coveralls]

Coverage decreased (-0.5%) to 92.455% when pulling a6f45f7 on Pchelolo:docs2 into 6bb41c0 on wikimedia:master.

[coveralls]

Coverage remained the same at 92.909% when pulling a8a1b06 on Pchelolo:docs2 into 6bb41c0 on wikimedia:master.

[coveralls]

Coverage remained the same at 92.909% when pulling 5eacfad on Pchelolo:docs2 into 6bb41c0 on wikimedia:master.

[coveralls]

Coverage decreased (-0.06%) to 92.844% when pulling b44378f on Pchelolo:docs2 into 6bb41c0 on wikimedia:master.

[coveralls]

Coverage decreased (-0.06%) to 92.844% when pulling b44378f on Pchelolo:docs2 into 6bb41c0 on wikimedia:master.