Add a CODEOWNERS file to get auto review requests

[lydiascarf]

Adds Bryan and I as CODEOWNERS, to be automatically requested on new PRs. See here for what it'll look like if you open a PR against main after this gets merged. Particularly, you should look at the "Reviewers" section and note who will be requested. I also pushed a test branch here where you can see that @jrmidkiff will be additionally auto requested for changes to the city org.

Open Questions:

• @BryanQuigley I'd be curious to know if, when you follow the above link, it says that I'll be requested and you won't. I see it as you being requested and me not being requested. I assume that's how it works, but I just want to double check.
• We don't have a team so we may not be able to do the round robin thing we set up when we were at Azavea. See docs here for that process.
• Finally, I'm not sure the best way to assign someone from the city as a CODEOWNER on their datasets since those files would have to be matched by filename patterns. Could we put the city datasets in a subdir? I'd rather not enforce a naming scheme, but I think it would be smart to prove out a process for this kind of thing if we are going to pick up more contributors

[BryanQuigley]

All of it works as expected.

I like the idea of a folder for the city of phila datasets - but I haven't looked into implementation details. We don't want any actual paths to change on the site.