Fix broken links on developers hub page

[seviu]

Going to the developer website (I am diving onto Starknet currently) I noticed the Developers Hub has most of the links broken, plus references to the old CLI (also broken) which probably don't make sense anymore.

I am not sure if this has to be changed through some CMS or if just having the right links is good enough. Forgive my naivety with this change and if there are better ways to perform this change, just feel free to close this PR and do it the proper way. Just bringing awareness to these broken links is good enough for me.