documentation for group/project level secret mount installment

[M0roSan]

I've tested at instance level where an admin token is used. This allows to create project access token for any projects.

For SaaS or limited use, this secret mount should work for

• group maintainer or higher access token for projects under a group
• project maintainer or higher access token for a project

We'll need to test and document that situation where group maintainer token is used. it should be able to create project access token for projects under the group. it shouldn't be able to create project access token for a project outside of the group where the token configured in plugin doesn't have access to.