Recommended improvements for section 'Concepts' at Ribasim.org

[esmee1998mes]

1. Concepts - Introduction: The introductory text under 1. introduction is also very relevant for the overview, see issue Recommended improvements for current Section 'Overview' at ribasim.org #2788. Because it explains better what Ribasim can actually used for right at the start.
2. Concepts - Model concept: Suddenly here you speak of 3 layers, opposed to the 4 in the overview. Confusing. Alternative: mention coupling in overview, but not as a separate layer.
3. Concepts - Julia core: To me 'nested allocation' seems perhaps better fitted under the Allocation section, it namely also links to sections here such as Allocation Networks. I would perhaps also change the name implementation to Computation
4. Concepts - allocation: Good to refer to Section 1.3 under Reference (https://ribasim.org/reference/usage.html#sec-allocation-settings) to