Improve documentation for layout sets

### Description

The documentation for [layout sets](https://docs.altinn.studio/nb/app/development/ux/pages/layout-sets) is sparse and lacks proper explanations.

### Additional Information

- Rewrite this sentence: "Sentralt i løsningen er at man har flere layout-sets som består av en eller flere sider og configurasjon. Hvert layout-set består av tilsvarende filer som en skjema tjenester."
- Exclude "Sentralt i løsningen" (irrelevant) - just explain what a layout set is (improve explanation)
- Move this part one level above "oppsett" (e.g. with heading "Hva er layout sets?")
- For "oppsett", explain all the steps and requirements for layout sets
- Do different sets need separate data models?
- Is 'Task' required?
- Are all the layout pages duplicated across sets?
- Anything else?
- Provide example(s)
- Most basic configuration
- How can I define layout sets where only the first page is static?

Start with the existing layout sets documentation at docs.altinn.studio/nb/app/development/ux/pages/layout-sets and review its current explanation and “oppsett” section. Clarify the requirements around data models, Task, and duplicated pages, then add basic and first-page-static configuration examples. Done means the page explains layout sets clearly and answers each listed question.

Written by the indexing model from the issue text.