So what we're looking at is probably two or three systems here. I can't help you on what systems integrate.

Documentation

This is for general documentation. SOPs, etc.

I generally advise documentation be stored in Markdown. This is after decades of having to migrate systems or being locked into a documentation system because a migration path was too timely.

Markdown is ultra portable and easy to store/backup. You can get change management interfaces for it, or store it in Git if you have to.

Even something like mdwiki would do the trick in a pinch. The value you see out of this lies in how well you keep it organized and how well you use it.

Client Secrets

This will be served by a secrets manager like https://github.com/Infisical/infisical

I've never used it but I've seen it recommended and it seems like a good project.

I've used things like KeePass, Bitwarden andOnePass. My preference would probably be none of them, but I do like Bitwarden for a personal vault.

Asset Management

This should be handled by an asset management system. This is for things like licenses, hardware, etc.

Snipe-IT was actually my pick for this. Just for assets though, nothing else.