AGENTS.md: Add configuration instructions

[ogajduse]

This section deserves a small expansion. I'd suggest adding, for example:

• mention about Dynaconf
• the fact that we're using the Dynaconf Vault loader
• the fact that Dynaconf can be configured using environmental variables
• something else I did not even think of

Originally posted by @ogajduse in #20228 (comment)

Here is a PR adding configuration instructions. ColeHiggins2#219