Switch to a new theme for website

[jopperm]

The Jekyll theme used by clangir.org has vanished from the internet (see also: https://github.com/orgs/community/discussions/136677). We need to pick a new theme and update the configuration to be able to update the site again.

[Lancern]

I would suggest we align with MLIR documentation theme which can be found at this repo, but MLIR uses hugo and we use jekyll ...

[bcardosolopes]

I'm fine with migrating to hugo or something else, but someone else would need to do the work. In the meantime I'll try to pick something else just so that we can update the page. Thanks for reporting this

[bcardosolopes]

I tried to speculate a bit without deploying locally and it didn't work, will try a more reliable approach tomorrow

[bcardosolopes]

I'm going to setup this theme: https://just-the-docs.github.io/just-the-docs/
My personal opinion is that we don't need to be similar to MLIR's doc pages.

[bcardosolopes]

Fixed, I still need to figure out how to better use the theme, but at least it's working again!