For Codidact communities in general, this question has been asked on Meta: [Can you post a question just to answer it yourself?](https://meta.codidact.com/posts/290638) There are several answers there, all confirming that this is allowed and encouraged.
For Mathematics Codidact in particular, I see no reason that the answer would be different. The Mathematics community has a help page [How to ask a great question](https://math.codidact.com/help/how-to-ask) that, like all the other communities, mentions "writing a self-answered question to share that knowlege".
One other possibility is [Articles](https://math.codidact.com/help/articles), which are Codidact posts that do not have answers. These are often kept in a separate category. For example:
- The [Papers category](https://electrical.codidact.com/categories/35) on Electrical Engineering Codidact
- The [Recipes category](https://cooking.codidact.com/categories/29) on Cooking Codidact
New categories can be added by starting a Meta discussion to see if the community would like that to happen. For Mathematics I suspect the question and answer format will work well even for explaining things you don't need to ask about yourself, so I'm just mentioning the possibility of another category for completeness.