In case you're not aware, there's another list on SharedWeight for just this sort of purpose. However, there has been no activity on that list for quite a while:

I don't think there's anything in particular about a given web content management system (CMS) that makes it more suitable for a dance series or caller "topic", per se, vs. any other. I think the choice is more a personal one based upon your given situation, tech comfort and desired capabilities.

The fact that you asked for "open source" implies a level of understanding that is different from mainstream and most likely means the choice for you could/would be different than for the next person.

I've personally used hand coded html/php/css and two CMS packages (Wordpress and Website Baker) for my work with similar topics and organizations. I'm not an expert by any measure but have been successful in doing what I needed to in this area by using these tools in a suitable manner.

Hand coding provides the best flexibility at the cost of detailed work/knowledge. Arguably also the best security - as the attack vectors are greatly minimized. BTW, website attacks are very much a real thing - I see attempts in my server logs 24x7.

The two mentioned packages are both open source and typically free & easy to install under low cost linux-based hosting packages (usually a single "click" on a software package selection page).

Wordpress is *very* popular, has far more in terms of plugins that extend functionality (e.g.: do you need to accept comments? integrate a calendar? sell something? want dedicated post types?) and automation of component updates (i.e.: security/bugfix), but is more complicated to manage ("complexity begets power"). It is at it's heart a blogging platform but people bend it to be a website management package routinely.

Website Baker (and related forks) does basic website creation & maintenance very well and I have been successful in getting (mostly) non-technical folks to be able to maintain their own sites using it ("with simplicity comes clarity").

This is an area of tech where there are many other package options and each have their own adherents - so you could get an infinite number of recommendations from the faithful. A place to start to sort that out for me included https://www.opensourcecms.com/ (I haven't used it since making my choices years ago - beware the rabbit hole!).

If having a dedicated package hosted under your own domain name (e.g.: mydancegroupname.org) for your site is not important, then several of the free (but not necessarily "open source") tools seem to work well for others. Folks seem to use Tumblr, Blogger, Weebly, Wix, Google Sites, etc. all the time. These provide easy templates but also limit your flexibility in making it look/work exactly how you want (yet may also offer unique capabilities integrated with other products). For many the limitations aren't a significant issue - but can be for others. For instance, if it is important that your group's image is all under one brand/domain then you'll most likely want to host a package on your (own or shared) server/service so you can have your site and email use the same domain name. There are ways to split this up across servers/services but you ratchet up the required knowledge/complexity as you do. For some of these providers, a "premium" product is available at a fee which enables additional capabilities similar to the above.

There's a great range of possibilities here and I've deliberately constrained my answer (though it may not appear so!). I may have aimed too high or too low for your interest level with the above. I'm happy to talk with you more specifically about my experiences in this area (and your interest/needs) if that is helpful.

-Don