Docs: Add presentation slides and posters

[berntpopp]

Add a chapter with slides and posters for the project detailed by conference or event and date.

TODO:

• [x] set up a new section after/before references in the documentation point
• [x] add all available materials to highlight the workflow and progress of the project
• [ ] make iframe of the one poster a little bit wider and adjust for a good haptic without scrolling
• [ ] check if there is some material left to implement? --> maybe the CADS presentation @berntpopp?

[constantinwolff]

Fix the current error: "The page you requested cannot be found (perhaps it was moved or renamed). You may want to try searching to find the page's new location, or use the table of contents to find the page you are looking for."

--> it probably happened after the automated documentation build with GitHub actions