Document cgroup system metric providers

[davidkopp]

Closes #115

Part of #104

PR for updating the READMEs in GMT: green-coding-solutions/green-metrics-tool#1273

[davidkopp]

For easier review, the document pages of the cgroup system metric providers are almost equal to the ones of the cgroup container metric providers. There are three differences:

1. "It can be used for system monitoring and tracking background processes such as the desktop environment. It is also used to calculate GMT's overhead."
2. container ids -> cgroup name strings
3. In the "How it works" section there is only a reference to the corresponding section on the container page

[ArneTR]

Why did you also correct the typos, they were important!!! :) :) (irony end)

LGTM 👍❤️