Making an HTML website scalable in size

I'm a beginner to web design and I have no interest in taking very far - just for my personal website to display my electronics projects. I'm an engineer using the CLI a lot, what I'm saying is I like to keep things simple, hence I want a basic website of just images and text that I have full control over. Wordpress and the like does not interest me on so many levels.

The problem is I do not know how to scale the website. My website will be made up of about 20 pages all of the same style with a navigation menu at the left with links to all over pages. Now, I don't want to have to update the nav on all 20 pages when I add just one more. How do I get around this? I quite like the idea of just one html file and all the content written in seperate Markdown files. It would work by clicking the links in the nav which would load the relevant Markdown material. I would rather stick to just HTML for the web side of things but I understand this may well not be possible.

Thanks George