Set up initial packaging

[khaeru]

As discussed on Friday 02 June.

In addition, I:

• Configured this repo on IIASA ECE's ReadTheDocs account.
• Added it as a subproject of the message_ix project, i.e. https://docs.messageix.org/projects/buildings/.
• Added redirects to the above URL from
  • /buildings
  • /buildings/
• Opened issues for things we discussed that aren't addressed in this PR:
  • Set up continuous integration #2
  • Migrate code from message_data #4

How to review

@Zh-xy @amastrucci I ask for approving reviews from each of you to confirm that you:

• View the list of commits and messages
• View the changed files, and inspect the file layout on this branch.
• View the added and expanded documentation pages, and confirm they are clear about how to obtain and work with the code.

Please ask any questions if the above are not clear or you need further information.

[khaeru]

FYI also @glatterf42.

[amastrucci]

Thanks Paul, looks good to me! We can merge this to the main branch, but we will need to check later how to merge with the new developments we are working on.

[glatterf42]

If you have started branches from some commit at main before this PR was merged, you may now need to reconcile these branches with the latest changes. You could rebase your branches on to the new state of main or you merge the changes of main into your branches - this is mostly up to you. Merging will create an additional commit, while rebasing will keep the commit history linear (so the history of the repo is easier to understand). If you don't feel comfortable rebasing your branches, please just let me know :)