Restructure README files

[oleschwen]

Having created multiple application codes, the different README files have become somewhat redundant. Plus, some documentation is aimed at developers, some is for client nodes, some is for both.

AC:

• well-structured README.md files, linking to each other and to further instructions, for the existing application code and different target groups

[oleschwen]

This issue addresses part of #2 and follows #11 and #14.

[oleschwen]

I'll prepare a suggestion for how to structure these with a trade-off of having little redundancy and looking at few places.

[oleschwen]

Discussed in call 2024-10-28: for the time being, use a single README.md, we can split it once we roll MediSwarm out to a non-developing user site and see which information is relevant for them.