ENH: Provided links to source packs

doc/Build.md

+## Getting the code
+
+Links to all code packs are available on from https://dl.openfoam.com. For OpenFOAM-v2106:
+
+- Source: https://dl.openfoam.com/source/v2106/OpenFOAM-v2106.tgz
+- ThirdParty: https://dl.openfoam.com/source/v2106/ThirdParty-v2106.tgz
+
 ## OpenFOAM® Quick Build Guide
 
 Prior to building, ensure that the [system requirements][link openfoam-require]