set up readthedocs to show multiple versions of the docs, including the main branch

[edwardhartnett]

readthedocs can show multiple versions of the documentation. We have the current documentation (which we can label v1.0.0), we will have a new documentation release for every version release from now on.

Also it would be great if we could see the current documentation from the main branch. This would be useful for developers.

@chan-hoo I see that readthedocs is getting the docs from your fork, instead of the main repo. Can we change that?

[edwardhartnett]

OK, I believe @MatthewPyle-NOAA actually has to do this, because according to readthedocs/readthedocs.org#4065 says "host of guthub repository" can set webhooks.

In any case I don't have permissions on this repo for webhooks. In any case this should be set up and owned by the code owner, not me.

[edwardhartnett]

OK, this is now working well. Thanks @MatthewPyle-NOAA for setting this up!