docs(contributing): Add mdbook install note about toolchain

[Kissaki]

The difftastic Cargo.toml currently specifies Rust version 1.66 to be used.

When running cargo install mdbook in the repository directory or manual sub-directory, the installation fails.

With a more recent Rust toolchain active, which can reasonably be assumed outside of the repository directory, the mdbook installation works fine.

If the user does not have a more recent toolchain active outside of the directory, the note at least serves as a pointer to the issue.

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The error in question:

error: failed to compile `mdbook v0.4.40`, intermediate artifacts can be found at `[…]\Temp\cargo-install[…]`

Caused by:
  failed to select a version for `env_logger`.
      ... required by package `mdbook v0.4.40`
  versions that meet the requirements `^0.11.1` are: 0.11.5, 0.11.4, 0.11.3, 0.11.2, 0.11.1

  the package `mdbook` depends on `env_logger`, with features: `anstream` but `env_logger` does not have these features.
   It has an optional dependency with that name, but that dependency uses the "dep:" syntax in the features table, so it does not have an implicit feature with that name.

  failed to select a version for `env_logger` which could resolve this conflict