Update the Umbrella README

[matbmoser]

Tasks:

• Update main readme from umbrella

[evegufy]

Umbrella README.md should be updated to reflect the latest addition of the SSI DIM Wallet Stub (once #125 is merged). Especially in this sections:

• https://github.com/eclipse-tractusx/tractus-x-umbrella/blob/umbrella-1.1.0/charts/umbrella/README.md#note-for-r2405
• https://github.com/eclipse-tractusx/tractus-x-umbrella/blob/umbrella-1.1.0/charts/umbrella/README.md#data-exchange

As well as the postman collection with example requests for the data exchange should be added to the repo, in a /docs/api/postman directory. Please also add a docs/README.md (example here) shortly describing that the directory contains example requests for the data exchange.