It is against the laws of speedrun.com to leave here an invitation to a speedrun community in a discord server?

What? Several communities have discord servers, almost all of them are linked on the LB for the game - either in a forum post in the game's forums or on the left hand side bar. I'm afraid I don't understand exactly what you're asking in this scenario.

I think this is "hey come run my game, here's the Discord server." I don't think that's per se against any rules but it's kinda bad form unless it's relevant to the topic of discussion.