Implement layout

[maurolepore]

bslib has not only features for theme but also for dashboards and layout

https://rstudio.github.io/bslib/articles/dashboards/index.html

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From Anne's https://github.com/2DegreesInvesting/tiltWebTool/issues/21#issuecomment-2154980389:

If you look at the Theme demo, I think it all contains the components that we would need:

• on the top being able to spring from one tab to another
• Plots
• Tables
• Upload buttons

I think we don't need a dashboard but maybe a landing page would be nice so that you can click from the landing page to the tabs on the top.