SDP Training (Unix) Updates:
* Updated version (2021.1 -> 2021.2).
* Added a bit more detail on "4. Where are things?" for p4d_N symlink/script.
* Updated "5. SDP Shell Environment Management" to account for
new-in-2021.1 shell environment mechanism.
* Added new section "Upgrade of SDP" including slide with sample
procedure.
* Moved "SDP Structure Overview (DEMO)" slide to later in the deck.
* Added more detail on "Outer to Inner".
* A few other very minor text tweaks.