feat: add gtm custom events and gtm for develop

[krissvaa]

[peholmst]

Heads up! There is a bigger restructuring of the articles in the next branch coming up. Unless you want to end up in merge hell, I suggest you merge this PR as soon as possible.

[jouni]

My focus has drastically shifted from documentation, and I’m no longer driving this change (to get stats about what search terms visitors use in our docs). I’ll leave the branch around for while still, in case someone wants to pick it up.