Fix Learning gem5 building guide; merge with with Building guide
At present the learning gem5 building guide is out dated and contains some dead lines:
I believe there is a big overlap here between this tutorial and our more general building documentation:
A lot of this should probably be merged, with the tutorial pointing to the general building documentation. The "Common Errors" section should also be moved to the general building documentation. The "Your gem5 build" section should probably remain within the tutorial.