Add a central page for the workshop

[MakisH]

This adds a central page for the preCICE Workshop to permanently link to, explaining the concept and referring to past workshops.

I have also included some example feedback from 2023, but I guess there might be more fitting feedback to highlight, instead.

Open questions:

• Should we now have both preCICE Workshop and preCICE Workshop 2025 in the sidebar?
• Should we have a folder preCICE Workshop with overview and specific events, instead?
• Should we make a similar page preCICE in conferences, or make it another section here?
• Depending on the answer to the above questions: what happens to the Past events folder and the items listed there?

Closes #463

[uekerman]

Let's do two folders preCICE Workshops and preCICE minisymposia with a general overview page each?

[uekerman]

Very helpful restructuring 👍
Some minor things to still tweak below. Then, good to go from my side.