Update readme to conform with new repo and package structure

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d62aa0c) 33.76% compared to head (11f03fc) 33.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6 +/- ## ======================================= Coverage 33.76% 33.76% ======================================= Files 20 20 Lines 2375 2375 ======================================= Hits 802 802 Misses 1573 1573 ``` | [Flag](https://app.codecov.io/gh/aiidalab/ipyoptimade/pull/6/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | Coverage Δ | | |---|---|---| | [ipyoptimade-py-3.11](https://app.codecov.io/gh/aiidalab/ipyoptimade/pull/6/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `33.76% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#carryforward-flags-in-the-pull-request-comment) to find out more.

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