I think you already have the top categories right. from a new user perspective, they are not going to go digging very far, but more mature users will.
Think of the new user process like this, think of it as peeking in a door of a new pub/bar, you might peek in look around see what its like, see if they have what ya want, if you cant find it you might ask for a bit of help bounce back out a bit quick.
Thats the help on installation and all the electronics topics and guidance in the first forum section.
As you get more familiar with the place you start to go deeper inside, look around more, maybe chat with the guy at the dart board, throw something on the jukebox or even karoke.
This is exhibition, showing off and bonding getting to know the regulars a bit more. Thats how the exhibition section seems to flow naturally to me, just my opinion. not just heres the fix but look whats cool too, you dont have to dig deep to find it.
As an aside to the pub allegory, I also think if you go back and look at the old exhibition you will see that a tremendous amount of technical knowledge discussion happened in the exhibition threads as more people asked how something was done or critiqued and kibitzed the item being displayed, it didnt function as just a broadcast show and tell but more a show and discuss.
And i think that section, the learning and introduction section of USING ARDUINO lends it self to that, where as at the bottom it is more of a broadcast and regular hang out section.
Back to the allegory.....
Now the pub is feeling more natural the place is rockin to you, its your regular hang out, they know your name and that cranky bunch at the end of the bar that gave ya the hairy eyeball, when you walked into their bar and asked the name of the drink, which you now have named after you because its YOU RE DRINK, and because you are a regular, they dont intimidate you so much, in fact you can hold your own with them now.
Thats the community section, a little isolating and you need to grow into it, explore the site deeper to find it at the bottom, you have to explore some to get there, but it isnot quite the front door baptism by fire, dont you know how to use a search engine to find a drink nme, we knew all these drinks years ago mentality that sometimes arises in some pubs (and IRL in the the previous forum flow).
So in my pub/bar story you can see a natural process come out of your menu choices and it flows fairly well (with the exception of the topics section which is a bit redundant and I think confusing to the flow)
We ll call the topics section the side graffitti room in the pub story, not quite lit right, a bit off the beaten path,gets all kinds of stuff written all over it because its a bit out of the normal flow pattern, just seems a bit out of place.)
But I still believe the new user process would flow best if you moved the exhibition ( karoke ) up so more than the regulars can find it and enjoy it.
As to your best of, Im not sure if I understand how you mean it to work, but if it is anything like the news section of the old forum where it has to rely on say an editorial oversite, it may fall to similar fates, and it may be better to keep it less controlled (open sourced even 
) to keep the flow fully organic rather than having to be tended to a lot.
I hope at least some of this makes sense and isnt too esoteric an explanation of my opinion of why you should move the exhibition to the bottom of the Using Arduino and now in review possibly reconsider the TOPICS section.
The fact that I can find a real world interaction with a natural flow in the process to liken the menu and user flow to means you got good usability design involved in the final outcome, and as I said in my first post it all in all is pretty good all the way around for such a big leap.
Just thought the tweaks might help.
Hope they do, just my 2 cents
Boz