docs: Add general citation from ATLAS HL-LHC Computing CDR (#2101)

* Add general citation from 'ATLAS HL-LHC Computing Conceptual Design Report'.
   - c.f. https://inspirehep.net/literature/1845103
   - CDS: https://cds.cern.ch/record/2729668

Author: matthewfeickert

docs/bib/general_citations.bib

@@ -102,6 +102,17 @@ @article{Tastet:2021vwp
     year = "2021"
 }
 
+% 2020-09-03
+@article{ATLAS:2020pnm,
+    author = "{ATLAS Collaboration}",
+    title = "{ATLAS HL-LHC Computing Conceptual Design Report}",
+    reportNumber = "CERN-LHCC-2020-015, LHCC-G-178",
+    month = "9",
+    year = "2020",
+    url = "https://cds.cern.ch/record/2729668",
+    journal = ""
+}
+
 % 2020-06-20
 @article{Krupa:2020bwg,
     author = "Krupa, Jeffrey and others",