Add roadmap page to User's Guide sphinx docs

[kandersolar]

• [ ] Closes #xxxx
• [x] I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
• [x] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [x] Pull request is nearly complete and ready for detailed review.
• [x] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

I think it would be beneficial to keep a roadmap document in a more public-facing spot. Our sphinx docs get way more traffic than github does, so I think housing it on RTD will get it in front of a lot more users. My hope is that this will help keep the roadmap in front of mind, plus maybe attract new contributors. I also like the idea of having a clear process for proposing and accepting edits to the roadmap... just make a PR with the changes.

I took some editorial liberties when assembling this roadmap based on the previous ones. Feedback from anyone reading this is desired.

[mikofski]

Thanks @kandersolar maybe we can take a step further and put some release dates and a few loose goals and priorities for next few releases?

[adriesse]

Two thoughts:

• if this is a wish list, why call it a roadmap instead of a wish list?
• couldn't many of these wishes be turned into issues in which the wishes are also more fully described? Oh yes, you mentioned visibility...maybe we can have links to issues where the wishes are more fully described?