Hi,
As the title of this thread says, I'm keen to learn how to make plugins. Can anyone direct me to good/comprehensive resources online where I can start learning?
Thanks in advance,
Chienne

I can recommend you with C# basics

For very basic lvl of c# you can pass the course of SoloLearn (won't take much time).
I also suggest you to take a look at existing plugins, and then, when you start to develop something you will need to use some .net decompiler tool (I personally use ILspy and dnSpy, but many devs here suggest JustDecompile), to look into dynamic libraries of Oxide and game you make plugin/extension for. (~/Managed/  folder)
And for advanced level of c# dont forget about MSDN.

Our documentation is not a resource to learn programming and is focused exclusively on uMod.  That said, the web is replete with information on programming generally, and when writing plugins you will likely need familiarity with specifically  ..

1. C# (with Visual Studio) coding information/practices
2. Modern .NET Libraries (for recent games)
3. Legacy mono (~3.5 fork) Libraries (for older games)
4. Server-side game libraries from game developers (obtained via a decompiler if necessary)
5. Oxide/uMod Plugin API & Wrappers (as documented here)
6. Basic Unity scripting

Thanks very much to @Orange, @2CHEVSKII, and @Calytic for your advice. That gives me lots to start learning, I greatly appreciate it. :)