[docs] Add sources section to Project.toml (#289)

Explicitly specifying where the package should be installed from when building
the documentation will make this task easier with Julia v1.11+.

docs/Project.toml

@@ -7,6 +7,7 @@ HDF5 = "f67ccb44-e63f-5c2f-98bd-6dc0ccc4ba2f"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 MPI = "da04e1cc-30fd-572f-bb4f-1f8673147195"
 PDMats = "90014a1f-27ba-587c-ab20-58faa44d9150"
+ParticleDA = "61cd1fb4-f4c4-4bc8-80c6-ea5639a6ca2e"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 StableRNGs = "860ef19b-820b-49d6-a774-d7a799459cd3"
 Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
@@ -25,3 +26,6 @@ StableRNGs = "1"
 Statistics = "1"
 YAML = "0.4"
 julia = "1.7"
+
+[sources]
+ParticleDA = {path = ".."}