docs: add favicon

[maxrjones]

closes #3766 by adding favicon + config to mkdocs

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[maxrjones]

thanks for the reviews. Afterwards, I noticed that the default zarr favicon was much lower res than icechunks (32x32 vs. 96x96) and thought icechunk's looks better, so I made a 96x96 zarr favicon.

[maxrjones]

thanks for the reviews. Afterwards, I noticed that the default zarr favicon was much lower res than icechunks (32x32 vs. 96x96) and thought icechunk's looks better, so I made a 96x96 zarr favicon.

I didn't do the alpha right though 😞