Fixes broken links in latest docs version

[melissawm]

Description

Fixes a few broken links in the documentation (in the Best Practices docs for plugins, and the Annotation tutorial).

Type of change

• Bug-fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

References

Closes napari/napari.github.io#364

How has this been tested?

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• example: I check if my changes works with both PySide and PyQt backends
  as there are small differences between the two Qt bindings.

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
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