Citations missing to Libxc

[susilehtola]

Describe the bug

Citations to Libxc should be included in the ABACUS manual.

The code should also

• print out the employed version of libxc (xc_version())
• print out the literature reference to libxc (xc_reference() and xc_reference_doi())
• print out the references of the employed functional(s)

Expected behavior

No response

To Reproduce

No response

Environment

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Additional Context

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