Leave a breadcrumb towards bootstrap config documentation in bootstrap.toml

[moxian]

I was curious as to the possible bootstrap options I can put in config.toml, but had some trouble figuring it out. There is no obvious documentation in config.toml (obviously), the documentation in src/bootstrap/defaults is nice, but also rather sparse, by design.
I had to dive into the parsing code, and stuble upon a very helpful doc comment there to realize that config.example.toml exists, and that it does, indeed, answer all of my questions.

So I figured it might be worth making this journey a bit easier for future contributors and add mention the config.example.toml directly in config.toml.

Now, since #137081 is in-flight which would rename config(.example)?.toml to bootstrap(.example)?.toml, I figure it's better to wait until that one lands, and submit the "new", "correct" filename upfront, instead of landing config.toml now and updating it to boostrap.toml later.

@rustbot blocked #137081

[rustbot]

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