Add a search box to the docs.wandb.ai landing

[ijdoc]

After becoming familiar with the docs site, most users dropping into docs.wandb.ai will want to quickly search for documentation which may or may not span across both models and weave. Having a universal search box on the docs landing page would significantly increase developer convenience across both products, specially since most users won't be familiar with exactly what features belong to which product

[johndmulhausen]

Totally hear you. The only issue with doing this now is that the search box would give the impression that you'd be searching across the Weave and Models docs, but in actuality, the two products are on two separate docs sites with two separate search indexes. Once we are in a unified situation, we'll definitely do this. For now I'm marking this as blocked.