BC-5096: adds docs regarding project structure

[OliverHappe]

Short Description

I moved the docs from https://docs.dbildungscloud.de/pages/viewpage.action?pageId=220233889 into the project.

I also added a decision tree regarding a type and an matrix for allowed imports.

Links to Ticket and related Pull-Requests

• https://ticketsystem.dbildungscloud.de/browse/BC-5096

Checklist before merging

• QA: In addition to review, the code has been manually tested (if manual testing is possible)
• PO: Any deviation from requirements was agreed with Product-Owner / ticket author / support-team
• DEV: Every new component is implemented having accessibility in mind (e.g. aria-label, role property)

Notice: Please keep this Pull-Request as a Draft (or add WIP label), until it is ready to be reviewed

[pullapprove]

Hey @OliverHappe, thanks for the PR! The review will start once
the tests, CI checks and PR requirements (see checklist in your PR) have passed.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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