added tutorial on growth loops

[jamesefhawkins]

Changes

I think growth loops are relevant for PostHog to consider more closely. I wrote up a tutorial on measuring them as I thought about the topic.

This pull request:

• Adds a growth loop tutorial
• Updates the sidebar to add the relevant link

It makes me think in future it'd be nice to do tutorials on:

• Measuring Performance Marketing
• Measuring Content Marketing
• The article mentions sales driven growth engines, but I think this may not be best covered by PostHog's product

[joethreepwood]

I've added quite a bit into this article, as there are a few places where I didn't fully follow the explanation of a growth loop/virality as I've previously understood it.

There's also substantial crossover between some of the distinctions. The StackOverflow and TikTok examples are separated into different categorties, for example, but have the same growth loop: Users create content which attracts more users who create more content...

I also feel we could add more to this as a tutorial, at least as far as showing what an example dashboard would look like?

[charlescook-ph]

@jamesefhawkins do you still want to publish this one?

[charlescook-ph]

@andyvan-ph do we still want to publish this one?

[joethreepwood]

This hasn't seen action or follow-up in a long while. I added a lot of feedback previously, but I'm still unsure if it's in a good place. Now that tutorials lie with @ivanagas I'll tag him for review but would encourage us to either merge or close.

[ivanagas]

I think this could be a good piece, but needs a rewrite and more visuals so I am going to do that. I'll close this one and open a new PR.