Add readme / TOC

[meichthys]

The readme should act as a table of contents for the docs (see the original Trilium wiki for the table of content layout).

The order of the pages could be re-arranged if it makes sense to do so.

[root-hal9000]

I can take that on sometime next week.

[Alumniminium]

Thanks @root-hal9000, I've merged it into main.