Update the citation instructions in the docs

1. Update the cff file to be more inclusive.
2. Update the BibTeX entry in index.rst to match the one generated
   by GitHub.

Change-Id: I26d031b2128d1d4330623bcdace284ca9ffce9e1

CITATION.cff

@@ -7,6 +7,7 @@ authors:
   given-names: Sameer
 - family-names: Mierle
   given-names: Keir
+- name: The Ceres Solver Team
 version: 2.1
 date-released: 2022-03-28
 license: Apache-2.0

docs/source/index.rst

@@ -44,12 +44,15 @@ Cite Us
 
 If you use Ceres Solver for a publication, please cite it as::
 
-    @misc{ceres-solver,
-      author = "Sameer Agarwal and Keir Mierle and Others",
-      title = "Ceres Solver",
-      howpublished = "\url{http://ceres-solver.org}",
-    }
-
+  @software{Agarwal_Ceres_Solver_2022,
+    author = {Agarwal, Sameer and Mierle, Keir and The Ceres Solver Team},
+    title = {{Ceres Solver}},
+    license = {Apache-2.0},
+    url = {https://github.com/ceres-solver/ceres-solver},
+    version = {2.1},
+    year = {2022},
+    month = {3}
+  }
 
 .. rubric:: Footnotes