[Playground] [Task]: Support for a default category of an example

[alexeyinkin]

What needs to happen?

Currently an example can have multiple categories. In the frontend, it gets highlighted in all of them, and they all are initially expanded:

This is confusing. We need a notion of a default category for an example. If it is selected by URL or as a default one in its SDK, then only that category should be expanded. If a user explicitly selects an example in some category, then that category should be expanded by default instead.

Issue Priority

Priority: 3

Issue Component

Component: beam-playground

[alexeyinkin]

.remove-labels 'awaiting triage'