fix: update broken link to code of conduct
Created by: creachadair
The code of conduct lives in the about repository since c15a3db4. Point to the Handbook site rather than the source file.
Supersedes #7779.
Created by: creachadair
The code of conduct lives in the about repository since c15a3db4. Point to the Handbook site rather than the source file.
Supersedes #7779.