Right, this is what I suspected. When you interact with the webserver it is typically apache that is running. Please issue the command:
$ grep apache /etc/apache
apache:x:48:48:Apache:/usr/share/httpd:/bin/bash
- No need for privilege. If you see the output above it is pretty safe to assume that you are running the normal server.
When Apache receives a Dokuwiki request it runs the dokuwiki code,
as user apache. Therefore if you are wanting to update a file it is the
apache user which does the updating. Take
start.txt. The permission bits mean that user bitnami has read and write access so can modify the file. The group named bitnami only has read access, probably not relevant in this case. "others", that is anyone with access to the file, has read only access, so cannot change it. Apache is regarded as "others".
You need to change the ownerships for all the old files you have imported. Please take care, you will need privileged access. If you have access to a colleague or friend with Linux operations experience I would strongly advise asking them to assist or at least supervise given that you say you are "really not familiar with Linux".
I won't be able to assist further for a while, I'll be spending the rest of the day at the hospital and may be rather "out of things" for a few days afterwards.