add doc build workflow

[epassaro]

This PR addresses #398

Documentation will be deployed to https://conda.github.io/constructor using the gh-pages branch.

An example on my fork: https://epassaro.github.io/constructor.

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[mcg1969]

ooooh, nice. will take me a bit to get to this but I am so glad to see this

[epassaro]

This isn't going to be merged, right? Should I close? @mcg1969

[jezdez]

This isn't going to be merged, right? Should I close? @mcg1969

Hi there! Lemme jump in here quickly to say that we're currently considering how to handle documentation for the various projects under the conda organization to improve their discoverability and help conda users and developers to find what they are looking for. Until we've come up with an overarching plan in the next few weeks, I suggest to keep this PR open, thank you for your understanding!