Add info on setup of dev workflow to docs and restructure a bit

[cchwala]

I added today's step-by-step guide to set up the dev workflow to the docs. It will need some polishing. Potentially, the doc could also be restructured regarding the content of "contribution" and "development".

Things done in this PR:

• add step-by-step guide to set up the dev workflow to the docs (based on info from last meeting)
• add working toctree with captions
• move contribution file to docs dir
• add a bit of structure with preliminary content (installation...)

This PR partly implements #8

• [x] All code checks from pre-commit passed

[codecov[bot]]

Codecov Report

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

Comparison is base (a26632b) 100.00% compared to head (ec3b400) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 20 20 ========================================= Hits 20 20 ```

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