Improve dev docs on how to disable the password enforcement for public links

_Originally posted by @kulmann in https://github.com/owncloud/ocis/issues/8604#issuecomment-1985083188_

For this particular case I would suggest 2 things:
- deprecate the service vars because it seems like it only makes sense to have the global var. Otherwise you can shoot yourself in the foot very easily (like I did now).
- make sure that I find some info easily, which takes me by the hand and tells me "hey, there is password enforcement and a password policy for public links. to enable or disable password enforcement, set the env var `OCIS_....` to `true` or `false` respectively. If you want to tighten or loosen your password requirements, use the env vars `OCIS_....`.". You might argue that this belongs in admin docs, but I expect it in the dev docs already - where the env vars are defined. I don't need an exhaustive list of env vars. An FAQ with the most important things one might want to solve would be much more helpful.

I have two screenshots for you. The first one shows, that you don't find useful content when you search for "Password". The second shows, that the env var list in the frontend service suggests that the two env vars listed there can be used interchangeably - which now turned out as incorrect.