I want to create a webserver local-network accessible but without router. The idea is to connect a device (phone, PC) to the Raspberry over WiFi, and see the webpage on the Apache Server.
Is it possible?
Complicated multipart question. So let's break it down.
First, of course you can run a web server on your Rpi. Whatever else it is doing with its networking doesn't affect that part.
Then, you talk about adding a generic device. Your examples of a PC and a phone are dramatically different.
There are other posts on this web site that talk about making a Rpi media server. I strongly suggest you do more research before posting a question like this. Spend some time in the archives and you will likely find your answer.