This is step 1 of GH-705.

The preceeding commits are an import of the individual commits from
shadow.wiki, imported by:
 * Rewriting shadow.wiki's history in a local checkout to retroactive
   put everything in the subdirectory `docs`, using `git filter-branch`.
 * Adding the rewritten shadow.wiki as a remote to a local checkout
   of shadow.
 * Creating this branch at the top of the imported shadow.wiki history.
 * Rebasing this branch on shadow/master. This also serialized
   the branch/merge structure from the wiki history, which resulted in
   some conflicts that needed to be resolved. I'm not 100% sure I got
   those right, but validated that the final state matches the
   current state of the wiki (after one fixup noted below).

This commit performs the following fixups:
 * Deletes `docs/Preparing-your-machine.md`. It was originally deleted
   earlier in the wiki's history, but was spuriously carried forward
   due to a mistake I made in conflict reso...