fix: add missing favicon to the page

[maximilianruesch]

Adds a favicon to both the landing page and sphinx docs.

Before:

After:

Note that different repositories for the same overarching docs.quantinuum.com page solve this differently. For example, lambeq uses the same inclusion for the landing page:
https://github.com/Quantinuum/lambeq-docs/blob/40d8dc84cbd2c38f829d5a1270b45c4c43b393ec/landing/src/app/layout.tsx#L37
but references the base themes favicon via a relative import instead of a separate inclusion, based on a certain env variable being set:
https://github.com/Quantinuum/lambeq-docs/blob/40d8dc84cbd2c38f829d5a1270b45c4c43b393ec/docs/conf.py#L92-L98

Perhaps all this should be unified or even handled in the base theme, but for now the simplest solution is just copying the file into the sphinx docs.

[maximilianruesch]

Failure due to the rename, thus waiting on #30

[CalMacCQ]

Ah yes this failure seems to be caused by sphinx trying to crosslink to the hugr-py docs with intersphinx here. These are currently down so a warning is emitted. As we are running a strict build which treats warnings as errors then the build fails.