Fix broken cross links in docs

[lucyleeow]

Our doc build has the following broken link warnings:

/home/runner/work/docs/docs/docs/docs/developers/architecture/app_model.md:39: WARNING: 'myst' cross-reference target not found: 'appp-model-menus' [myst.xref_missing]
/home/runner/work/docs/docs/docs/docs/naps/9-multiple-canvases.md:168: WARNING: 'myst' cross-reference target not found: 'Implementation' [myst.xref_missing]
/home/runner/work/docs/docs/docs/docs/naps/9-multiple-canvases.md:224: WARNING: 'myst' cross-reference target not found: 'UI-Design-and-Architecture' [myst.xref_missing]
/home/runner/work/docs/docs/docs/docs/naps/9-multiple-canvases.md:78: WARNING: 'myst' cross-reference target not found: 'UI-Design-and-Architecture' [myst.xref_missing]
/home/runner/work/docs/docs/docs/docs/release/release_0_4_19.md:72: WARNING: 'myst' cross-reference target not found: 'Deprecations' [myst.xref_missing]
/home/runner/work/docs/docs/docs/docs/tutorials/processing/dask.md:207: WARNING: 'myst' cross-reference target not found: 'make-your-life-easier-with-dask-image' [myst.xref_missing]
/home/runner/work/docs/docs/docs/docs/tutorials/processing/dask.md:329: WARNING: 'myst' cross-reference target not found: 'brain_dataset_tutorial.md' [myst.xref_missing]

I've just used reference labels when the original title (?) labels have broken, reference labels seem easier and less brittle.

Note the last one, AFAICT there originally was a PR to add a brain_dataset_tutorial/md file: https://github.com/napari/napari.github.io/pull/64

But what got merged was: https://github.com/napari/docs/pull/132 where that example was just added to the dask.md file, so I've removed that reference from the list.

[psobolewskiPhD]

Thanks for the explanations @melissawm I think I lean to those fixes vs. the new references, because they are unlikely to be used outside the nap.

[lucyleeow]

@melissawm I will close this as you already address these in your #393 PR