Update CITATION.cff (#40363)

You will note the `Cite this repository` link is not working.

This commit fixes the underlying file...

* `authors` was not indented
* `authors` required by `preferred-citation`
* `authors` list required at top level (I simply duplicated)
* `"USA"` not correct country code
* `month` requires an integer month number
* Added URL to the actual pdf of the cited paper
* Used `identifiers` for doi and LLNL doc number
* added `abstract` copied from paper

Various fixes were confirmed by `cffconvert` using `docker run -v `pwd`:/app citationcff/cffconvert --validate`

CITATION.cff

@@ -27,11 +27,21 @@
 # And here's the CITATION.cff format:
 #
 cff-version: 1.2.0
+type: software
 message: "If you are referencing Spack in a publication, please cite the paper below."
+title: "The Spack Package Manager: Bringing Order to HPC Software Chaos"
+abstract: >-
+  Large HPC centers spend considerable time supporting software for thousands of users, but the complexity of HPC software is quickly outpacing the capabilities of existing software management tools.
+  Scientific applications require specific versions of compilers, MPI, and other dependency libraries, so using a single, standard software stack is infeasible.
+  However, managing many configurations is difficult because the configuration space is combinatorial in size.
+  We introduce Spack, a tool used at Lawrence Livermore National Laboratory to manage this complexity.
+  Spack provides a novel, re- cursive specification syntax to invoke parametric builds of packages and dependencies.
+  It allows any number of builds to coexist on the same system, and it ensures that installed packages can find their dependencies, regardless of the environment.
+  We show through real-world use cases that Spack supports diverse and demanding applications, bringing order to HPC software chaos.
 preferred-citation:
+  title: "The Spack Package Manager: Bringing Order to HPC Software Chaos"
   type: conference-paper
-  doi: "10.1145/2807591.2807623"
-  url: "https://github.com/spack/spack"
+  url: "https://tgamblin.github.io/pubs/spack-sc15.pdf"
   authors:
     - family-names: "Gamblin"
       given-names: "Todd"
@@ -47,12 +57,32 @@ preferred-citation:
       given-names: "Bronis R."
     - family-names: "Futral"
       given-names: "Scott"
-  title: "The Spack Package Manager: Bringing Order to HPC Software Chaos"
   conference:
     name: "Supercomputing 2015 (SC’15)"
     city: "Austin"
     region: "Texas"
-    country: "USA"
-  month: November 15-20
+    country: "US"
+  month: 11
   year: 2015
-  notes: LLNL-CONF-669890
+  identifiers:
+    - description: "The concept DOI of the work."
+      type: doi
+      value: 10.1145/2807591.2807623
+    - description: "The DOE Document Release Number of the work"
+      type: other 
+      value: "LLNL-CONF-669890"
+authors:
+  - family-names: "Gamblin"
+    given-names: "Todd"
+  - family-names: "LeGendre"
+    given-names: "Matthew"
+  - family-names: "Collette"
+    given-names: "Michael R."
+  - family-names: "Lee"
+    given-names: "Gregory L."
+  - family-names: "Moody"
+    given-names: "Adam"
+  - family-names: "de Supinski"
+    given-names: "Bronis R."
+  - family-names: "Futral"
+    given-names: "Scott"