Add a dedicated, comprehensive "Permission levels" page

[ivanaur]

Roles/permission levels exist on several levels within the organization. They also exist on multiple pages in the Semaphore docs:

1. Organization role
2. Group role
3. Project role

What should be added:

• How does each role level correlate to their roles on GitHub (eg. does "Organization Admin" on Semaphore overwrite the "Read" permission level on GitHub)
• Expand on "everything" ie. how do roles correlate with different features such as "Secrets", "Artifacts", etc.
• Can Organization role overwrite the Group role or Project role and vice versa

[TomFern]

@ivanaur good idea! I created an issue to track it: #87