Rename navbar entries (API and Contribute)

[TimMonko]

References and relevant issues

napari/napari-sphinx-theme#200

Description

Shortens the name from "API Reference" to "API" with the hopes that this was enough to prevent the wonky navbar, however, it was not enough so we would need to modify the max-width in napari-sphinx-theme (or remove napari-hub as a link). I think from a content perspective "Reference" is redundant as a navbar name because all docs are reference material, so "API" (Reference) is implied.
It's quickly grown on me and I quite like it actually Compare how "busy" the current navbar looks on napari.org

Q: Should napari-hub be removed? We link to it in the plugins page, and could make that more prominent in the plugin page, or go back to linking it more prominently on the homepage.

And just to show a different solution that is left aligned:

[willingc]

Yes please! Thanks @TimMonko

[TimMonko]

Thanks @willingc
Another side thought (I can open an issue if this gets merged)
What about renaming Contributing -> Contribute? My gut impression is that it is more welcoming, but I know there are schools of thought on this that I'm not too caught up on

[willingc]

Makes sense to me to rename to Contribute.

Also, supported by Google's style guide: https://developers.google.com/style/headings

[TimMonko]

Thanks @willingc . I renamed to contribute since you were the original PR approval.

I will make a follow-up to propose removing napari-hub from the navbar, but would like to also improve the plugin index to make this a more obvious path to go to

[TimMonko]

This seems to fix napari/napari-sphinx-theme#200 now with Contributing -> Contribute as well

[TimMonko]

@napari-bot make docs

[TimMonko]

@napari-bot make html