[DOCS] Improve documentation to include setup instructions

[LennartCode]

Currently, just installing Syncthing and enabling the extension is not sufficient to set up the plugin. One has to update the PATH (on Windows at least, don't know about other OS's), copy the API key and depending on the Syncthing setup change other settings, which are currently undocumented. As a lot of Obsidian users are not very tech-savvy, I would like to add documentation to help setting everything up for a standard Syncthing installation. Please let me know, if this would be a welcome contribution :)

PS: There wasn't a template for documentation issues, so I made this up. I hope this is okay.

[LBF38]

Hi, thanks for opening an issue on documentation. Feel free to contribute on this topic if you've got time.
Thanks in advance !