Add favicon and configuration

[aganders3]

Migrated from napari/napari#5182

This adds support for showing a napari logo as a "favicon" (browser tab icon) on the docs page.

Based on discussion in #4897 a logo silhouette was preferred over full-color. This PR adds a few assets and configures them in the header according to what I could find for current favicon best practices:

• An SVG that detects light mode and dark mode and switches the fill color accordingly (for Chome, Firefox, etc.)
  • <link rel="icon" href="_static/favicon/logo-silhouette-dark-light.svg" type="image/svg+xml">
• A 192x192 black-on-transparent PNG (for Safari) - setting sizes="any" is needed for Chrome to prefer the SVG
  • <link rel="icon" href="_static/favicon/logo-silhouette-192.png" sizes="any" type="image/png">
• A 180x180 full-color PNG with no border for the “apple-touch-icon” (icon for iPhone/iPad “Save to Home Screen” and used by others)
  • <link rel="apple-touch-icon" href="_static/favicon/logo-noborder-180.png" sizes="180x180" type="image/png">

macOS Safari and Chrome Dark Mode

macOS Safari and Chrome Light Mode

iOS “Save to Home Screen”

N.B. this also adds a dependency on sphinx-favicon for the documentation building step.

References

closes napari/napari#4897

https://stackoverflow.com/questions/54639192/change-the-favicon-of-the-sphinx-read-the-docs-theme/54665517#54665517
https://evilmartians.com/chronicles/how-to-favicon-in-2021-six-files-that-fit-most-needs

How has this been tested?

Tested documentation building and serving locally.

[psobolewskiPhD]

I posted this in the previous PR in napari repo.
To me everything looks great! Love it, long overdue!
Code changes make sense based on the docs at https://github.com/tcmetzger/sphinx-favicon
I wish the PR docs comment thingie was working so everyone could more easily see the icons.
Ultimately, I think this one comes down to @jni though, especially given NAP-5 (https://github.com/napari/napari/blob/06a90c7469b20bcd4c741fa84bafee9704b981f0/docs/naps/5-new-logo.md)

[jni]

Nope, you have equal rights here @psobolewskiPhD, NAP-5 is only a proposal for now. ;) This should come in either way — if NAP-5 is accepted (and I think it'll require a lot of iteration before it is), then we can update all this stuff.

Thanks @aganders3!