[IMPROVEMENT] Add "updating docs" to how to contribute guide

[petersalemink95]

In the How to contribute guide, we have a list with potential ways of contributing. You can also contribute by updating the documentation. Update the list in https://power-grid-model.readthedocs.io/en/latest/contribution/CONTRIBUTING.html#ways-of-contributing by adding "updating documentation"

[mgovers]

PowerGridModel/.github#64 will solve this.