Add AGENTS.md and docs on how to use AI agents (#58561)
AGENTS.md has instructions to various AI agents on how to handle various
tasks. The current contents are preliminary, I haven't really verified
that they work well yet (there's still various other issues I'm working
through), but I wanted to get the basic structure in place. In addition,
add documentation to the Contributor's Guide on how to set up and
configure various AI agents to work well with Julia.
