docs: update docs for trg1

[Phil91]

Description

Adjusted documentation structure

Why

To have a matching documentation structure and align with trg 1

Issue

Refs: #247

Checklist

Please delete options that are not relevant.

• I have followed the contributing guidelines
• I have performed IP checks for added or updated 3rd party libraries
• I have created and linked IP issues or requested their creation by a committer
• I have performed a self-review of my own code
• I have successfully tested my changes locally
• I have added tests that prove my changes work
• I have checked that new and existing tests pass locally with my changes
• I have commented my code, particularly in hard-to-understand areas
• I have added copyright and license headers, footers (for .md files) or files (for images)

[evegufy]

could you please also mention the API Hub https://eclipse-tractusx.github.io/api-hub/ssi-credential-issuer?

[Phil91]

added