chore: release v0.3.0-alpha.1

[aborgna-q]

With the rename of the crate from hugr_quantinuum to hugr (#914) we broke release-plz' automatic changelog generation (since it cannot find the crate anymore), and all links to crate-related urls.

This alpha release should fix those issues, in addition to letting us claim the new crate name.

I added the partial changelog bits from #898, #893, and #840 since they won't be included in the new package's PRs.

drive-by: Move CHANGELOG and README from the root to the hugr dir to be more consistent with other packages in the workspace, and symlink them instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.63%. Comparing base (0565fee) to head (1a5e185).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #916      +/-   ##
==========================================
+ Coverage   85.56%   85.63%   +0.06%     
==========================================
  Files          78       78              
  Lines       14444    14444              
  Branches    14444    14444              
==========================================
+ Hits        12359    12369      +10     
+ Misses       1445     1436       -9     
+ Partials      640      639       -1     

Flag	Coverage Δ
rust	85.63% <ø> (+0.06%)	⬆️

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