fix: Move declarative.yaml example to the hugr-core crate

[aborgna-q]

This is a file used for the hugr-core docs, so it should be included in that crate.

When trying to cargo publish, I got an error due to it not getting packaged.

error: couldn't read src/extension/../../../hugr/examples/extension/declarative.yaml: No such file or directory (os error 2)
  --> src/extension/declarative.rs:12:10
   |
12 | #![doc = include_str!("../../../hugr/examples/extension/declarative.yaml")]
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `include_str` (in Nightly builds, run with -Z macro-backtrace for more info)

[doug-q]

If I understand, cargo publish requires the crate to build "isolated" from the workspace?

Can we build "isolated" in CI?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.78%. Comparing base (d89b6f1) to head (bef51df).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1131   +/-   ##
=======================================
  Coverage   86.78%   86.78%           
=======================================
  Files          91       91           
  Lines       18560    18560           
  Branches    18167    18167           
=======================================
  Hits        16107    16107           
  Misses       1606     1606           
  Partials      847      847           

Flag	Coverage Δ
rust	86.86% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[aborgna-q]

Can we build "isolated" in CI?

It'd be nice! Can you open an issue?