chore: Don't produce docs for hugr binary target

[aborgna-q]

This avoids the error due to duplicated target names, as the binary hugr has the same name as the main library hugr.

cargo doc showed this warning:

warning: output filename collision.
The bin target `hugr` in package `hugr-cli v0.1.4 (/home/runner/work/hugr/hugr/hugr-cli)` has the same output filename as the lib target `hugr` in package `hugr v0.8.0 (/home/runner/work/hugr/hugr/hugr)`.
Colliding filename is: /home/runner/work/hugr/hugr/target/doc/hugr/index.html
The targets should have unique names.
This is a known bug where multiple crates with the same name use
the same path; see <https://github.com/rust-lang/cargo/issues/6313>.

This caused the resulting docs to produce non-deterministic output, with either target overriding the other.
See cargo#6313.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.24%. Comparing base (afe1a77) to head (19149a3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1313   +/-   ##
=======================================
  Coverage   87.24%   87.24%           
=======================================
  Files         107      107           
  Lines       19586    19586           
  Branches    17322    17322           
=======================================
  Hits        17088    17088           
  Misses       1714     1714           
  Partials      784      784           

Flag	Coverage Δ
rust	86.69% <ø> (ø)

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