Discussion
I have several systems that I want to synchronize between my phone, my laptop and a tablet. Many of those systems have their own way of synchronization, i.e. Obsidian's Obsidian Sync. Systems like that work fine, but there are two disadvantages: First, I don't want to have to maintain multiple systems if I don't have to. Second, I want to be in control of how I back them up. I realized that if I can sync them to the Raspberry Pi, I can then back them up from there.
After looking a a few alternatives, Syncthing (and syncthing-fork for android) seemed to be the clear winner. It was a bit of a stretch to get my head around how to make it work on all my devices, but the Getting Started page helped.
Please note that this page is only for the initial set-up of Syncthing on the Raspberry Pi. See also the initial set-ups for my Laptop and Phone. I cover the steps to share individual folders in their respective sections, i.e. Self-hosting/4. Note-taking and Laptop/4. Note-taking.