W1D2

[glibesyck]

Intro:

• added prereqs description.

T1:

• fix links to models;
• fix output errors.

[glibesyck]

T2:

• fix output error (plots were shown on the untrained network as loss function is student's exercise).

T3:

• changed exercise parameters & wordings.

Conceptual question to T3: most likely, we would like to preserve pR + pL = 1; thus, the second slider in the interactive demo should be removed.