refactor data source documentation

[andreasgerstmayr]

The documentation pages for each data source (e.g. https://grafana-pcp.readthedocs.io/en/latest/datasources/bpftrace.html) should be in a how-to/guide style with screenshots, not a reference style with very sparse information.

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This issue has been automatically marked as stale because it has not had recent activity in the last 60 days. It will be closed in 30 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity in the last 90 days. It will be closed in 60 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity in the last 90 days. It will be closed in 60 days if no further activity occurs. Thank you for your contributions.