Update tutorial numbers and names

[tim-shea]

Objective of issue: Update tutorial numbering across Lava and Lava libraries to avoid duplicate numbers.

Lava version:

• [X] 0.6.0 (feature release)

I'm submitting a ...

• [X] documentation request

Current behavior:

• Tutorials in lava, lava libs, and lava-loihi are referred to by numbers, which start at 1 in each subdirectory of tutorials. This can be confusing because "Tutorial 1" does not refer to a specific, identifiable ipynb, nor does the numerical sequence indicate anything useful about the prerequisites for each tutorial.

Expected behavior:

• Tutorials could not be numbered and just use informative names (as in lava-dl), could be numbered in a consistently incrementing sequence across all folders, or could use a letter+number sequence (e.g. L1 for learning 1, D3 for dl 3).