Add (free text) search function on landing page

[han-f]

Use-case: I want to learn about whether institution X contributed something to the OEP, or whether there is anything related to data Y on the OEP.

Then I think it would be useful for me to find a free-text search bar right on the landing page where I can search without having to navigate to https://openenergy-platform.org/dataedit first.

I would then also like the search results to include links to any factsheet that may be answering to my search query.

cc @stap-m @l-emele

[han-f]

layout-wise this search bar could be placed quite prominently under / over the four boxes.

[christian-rli]

Building this should probably also close #294