PR for updating docs theme

[nucleosynthesis]

This PR modifies the theme to be more like those used in other CMSSW codes (eg the CAT doc pages). No need to merge until we're ready to make that change.

[nucleosynthesis]

many of the changes are already merged into the main branch - only changes to mkdocs and index are relevant

[kcormi]

Hi Nick,

I tried out this theme and the build, it looks good to me, I think its a nice a nice improvement over the current one. And all the various boxes/features etc seem to be working.

The only small thing I noticed, is I think in the current PR it is using an old version of the logo? The one on the pages has the fairly saturated colours, where I thought we were switching to the less saturated version.

NB: I didn't check to make sure the content is unchanged, I have only been checking for style so far.

[nucleosynthesis]

Hi @kcormi - thanks, yeah the logo is a little outdated, currently getting the latest and greatest one from Clemens to add in.

[kcormi]

I just rebased this PR on top of the main, to see the changes more clearly.

The only conflict was in the physicsmodel.md, which is now consistent although I don't know if we prefer to use $\vec{\mu}$ instead of $\vec{\theta}$ since I think the first one is consistent with the paper. This could also be updated in another PR, where we might want to check the notation and make it as consistent as we can though.

@nucleosynthesis, please just check that nothing got missed or changed in an unintended way.

[nucleosynthesis]

Looks good to me - I think probably going with the \mu in the physics model is better since we use that in the paper (and in the rest of the docs I already tried to go through and update to use the paper notation where I could)

[kcormi]

Looks good to me - I think probably going with the \mu in the physics model is better since we use that in the paper (and in the rest of the docs I already tried to go through and update to use the paper notation where I could)

Thanks, I've updated it to use $\vec{\mu}$ and $y$.

From my side the PR looks ready.

[nucleosynthesis]

I'd be happy to have this one merged, since others are making edits to the docs (from the CAT hackathon), we should probably get this one in so we don't have conflicts later?