How can I share an iCloud photo album? I just created three albums at iCloud.com that I'd like to give other people read access to. The information here describes only how to create a new shared album, but the albums I want to share have already been created. 

The people I'll be sharing with don't have Apple devices.

I'm used to cloud storage facilities such as Google Drive or Dropbox that make it easy to create a link to something in the cloud so that the link can be sent to others. Isn't there a way to do this for iCloud?
You will need to create the album with iCloud Photo Sharing in order to do that. iCloud will not let you share a personal album that you created as they are only a means of organizing media. 

With iCloud Photo Sharing, you can easily accomplish your goal and share the album either with specific email addresses or you can share a link, if you make it public. 

I suggest selecting all of the photos in the album that you have already created and choosing to share them to the iCloud Photo Sharing album that you create in order to publish those photos.