link fixes 2

[davstrom]

Links with "intermediate" in them are likely to fail, but this is still worth testing. There seems to be a problem with Gitbook when a markdown file and a directory have the same name and are in the same directory. I changed chapter1/intermediate.md to chapter1/intermediate/README.md, but that did not work on my local copy of gitbook. We can see if it helps in this PR.

[claassistantio]

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