[IMPROVEMENT] Add section on PowerGridModelInterface to docs

[vincentkoppen]

Problem faced

With the current docs we briefly mention the PowerGridModelInterface in the Quick start and some examples, but this deserves a more prominent spot. Since almost all users of PGM-DS will use it together with PGM.

What we currently have:

• quick start: https://power-grid-model-ds.readthedocs.io/en/stable/quick_start.html#performing-power-flow-calculations
• existing example: https://power-grid-model-ds.readthedocs.io/en/stable/examples/pgm/basic_pgm_examples.html

(Seperated from the discussion in #288)

Improvement proposal

Add a new section to the docs on PowerGridModelInterface that is directly between Model Interface and Examples in the left bar of our docs.

The first version can be brief:

• Show how to use it to perform a calculation
• How to go from grid -> input_data
• How to go from input_data -> grid

Community Meeting

• I wish to discuss this improvement in the bi-monthly PGM community meeting. Please join the PGM community meeting.