Update docs to fix various 404s

Found a few 404s that seemed like simple fixes:

The Result docs use old_io Writer as an example. Fix the link to old_io Writer. There's probably an effort to update the example away from a deprecated api but this was a simple fix.

rustc/plugin was pointing at the old guide and it was a broken link anyways (plugin vs plugins). Point at the book instead.

The main page of the API docs referenced c_{str,vec}. Looks like these were deleted in 25d5a3a19423fee01787de87a56d185dd4e0a4e7. Point at ffi docs instead.

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