Add citation to Libxc in the manual's introduction

[susilehtola]

Description

This adds the missing citation to Libxc in the manual's introduction.

User API & Changelog headlines

• RN 1
• RN 2

Dev notes & details

• Feature1
• Feature2

Questions

• Question1

Checklist

• Tests added for any new features
• All or relevant fraction of full tests run

Status

• Ready for review
• Ready for merge

[loriab]

Good idea to include the citation at the front of the manual, too, not just in code output.

I might have said "many", not "most", but upon reflection, "most" is probably right, both by selection volume and choices for energy() argument. :-)

[loriab]

/home/runner/work/psi4/psi4/code/objdir/doc/sphinxman/source/introduction.rst:141: ERROR: Unexpected indentation. appease the Sphinx