Drop docs/docsite/requirements.txt

[oraNod]

This PR resolves #107 and is a follow up on #60 and #271

Remove docs/docsite/requirements.txt in favor of the relaxed requirements file.

[oraNod]

I've been wrangling with this one a little more and thought I'd just send a PR and ask. Hopefully this is a sensible way to provide the equivalent of docs/docsite/requirements.txt (allowing the ability to test different packages and versions on the devel branch).

It seems there might be two ways to go about this.

1. Either install requirements.in or requirements-devel.in.
2. Install requirements.in and then requirements-devel.in on top of the main dependencies.

Option two should make it possible to reduce requirements-devel.in down to two packages: sphinx-intl and pygments.

[webknjaz]

2. Install `requirements.in` and then `requirements-devel.in` on top of the main dependencies.

It must be a single command or, alternatively, both commands need two -c so that the constraints remain the same.
If you run one, then the other, then, being generated separately, they may have different versions pinned and the second one may upgrade, or downgrade dependencies installed by the first one.

Option two should make it possible to reduce requirements-devel.in down to two packages: sphinx-intl and pygments.

Try adding -r requirements.in to requirements-devel.in.

[samccann]

Verified on Jenkins build that the docs build w/o this file.