Looks cool, but it has some small issues about layouting, and I would give some suggestions.
First Why the panel at the left is not fixed?! When user scroll the content panel/page the navigation panel also scrolls. I think it should be supposed fixed.
But for mobile, it’s really great. I see it goes at the top, and it’s togglable, very good.
For mobile you may either set the top navigation fixed, or have a quick button at the bottom of page for instance to jump to top.
In any way, user should access the navigation panel very easy without scrolling too much.
I see you apply some background colour for code snippet sections, very good. Bu I don’t know why did you not let them display as a block?! let them fit the parent bound. (remove theat width:50% from .code-section rule)
For mobile a little more line-height could be great. Also add a little more space between each paragraph.
I see you place <br/> tag inside <p> paragraph tags. I suggest instead of adding <br/> you close the paragraph, and start another one. Except when two sentence are really related to each other.
If you could caption each code section and images could be awesome.
Adding a horizontal line <hr/> after each title in reading panel could be great.
I think if you could center images could be better.
Also use inline-code too. In your description, whenever it’s about keywords or any inline code, make it as inline code too. for example considering following paragraph
Typically if we had all of our items set to flex: 1 1 200px and then wanted one item to grow at twice the rate, we would set that item to flex: 2 1 200px. However you could use flex: 10 1 200px and flex: 20 1 200px if you wanted.
Note the inlined section. also let inlined codes come with a background colour just like you did for block codes.
Very good overall, like to see some progress and update about the fixed navigation soon.
Keep goin on great work, happy programming.