Prepare repository and package for retirement

[willGraham01]

Prepares the brainreg-napari package for deprecation following the upcoming merge of brainreg and the napari plugin.

What does this PR do?

• Edits the README to point to the new package
• Adds a deprecation warning when the package is imported
• If install is attempted, fixes brainglobe tool versions <1 to attempt to avoid conflicts.

See brainglobe/BrainGlobe#37 for more details.

Documentation changes are staged in the following PR:

• on bg website repo

Upon merging this PR, we will:

• Tag a new version, v0.9, and push this to PyPI.
• Mark the package as deprecated on PyPI.
• Move outstanding issues to brainreg repository.
• Archive this repository

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (71f50d2) 76.36% compared to head (087f004) 76.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
+ Coverage   76.36%   76.53%   +0.16%     
==========================================
  Files           4        5       +1     
  Lines         292      294       +2     
==========================================
+ Hits          223      225       +2     
  Misses         69       69              

Files	Coverage Δ
brainreg_napari/__init__.py	100.00% <100.00%> (ø)

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[alessandrofelder]

Awesome.