[Task] Trim Readme in `identity_iota` to reduce duplication

## Description
Currently, the root level README file and the README in `idenity_iota` are duplicated. This is required to have a proper crate documentation in `idenity_iota` AND have a good entry point on GitHub.

We could add an automatic step during crate publishing, that just the root level README into the `identity_iota` folder.

## Motivation
This way we could reduce the `identity_iota` README to a much shorter version and avoid having the README in two places, possibly diverging over time.

Review the root-level README and the README in `idenity_iota`, then inspect the crate-publishing step that could copy the root README into the crate folder. Done means the crate README is shortened without losing its documentation entry point, and publishing keeps the two locations from diverging.

Written by the indexing model from the issue text.