Migration to docsy theme

[yurishkuro]

Jaeger website today is using a completely custom theme. This creates a maintenance headache as we run into various layout bugs (e.g. #744, #745). We also have issues with some old dependencies for CASS like bulma (#730).

I looked around in other CNCF projects and noticed that both OpenTelemetry and Kubernetes websites are using https://github.com/google/docsy theme, which is likely much better supported.

Of course, it's not a very trivial job to replace the theme, as we also have plenty of CSS customizations that we would want to keep. So before we really decide to do that we need some investigation, e.g. just trying out enabling the docsy theme and spot-checking what would break in Jaeger as a result, and collect the findings into a report.

One thing we'd need to do is customize the color schemes to match the existing Jaeger website (https://www.docsy.dev/docs/adding-content/lookandfeel/)