Fix#13965.
This commit adopts the second strategy I outlined in #13965, where the bulk of the code is still "smoke tested" (in the sense that rustdoc attempts to run it, sending all of the generated output into a locally allocated `MemWriter`). The part of the code that is ignored (but included in the presentation) is isolated to a three-line `main` function that invokes the core rendering routine.
In the generated rustdoc output, this leads to a small break between the two code blocks, but I do not think this is a large issue.
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