Add a link to CI documentation in rustc-dev-guide

[Kobzol]

I recently rewrote and updated the documentation of rust-lang/rust's CI in the rustc-dev-guide (PR, documentation).

I don't think that it's productive to have the documentation on two places, because it can get out of sync (which is exactly what has happened to the Forge and rustc-dev-guide versions recently). So this PR removes the CI documentation from the Forge and replaces it with a link to rustc-dev-guide. I took all the content from the Forge (that was still relevant) and added it to the dev guide, so there should be no documentation lost.

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[kennytm]

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