DOC: Updating technique docs

[rlundeen2]

Updates the attack techniques documentation (doc/code/scenarios/0_attack_techniques) to match where techniques actually live today.

• Rewrites the "Where techniques come from" section to describe the setup group-module layout (pyrit/setup/initializers/techniques/: core.py, extra.py, airt.py) and how TechniqueInitializer aggregates groups and injects the group name as a selectable tag.
• Documents the tags selection (core default / ["core", "extra"] / ["all"]) and lists the full catalog.
• Adds an end-to-end example that pulls a registered technique from the registry, binds it to a target + scorer via create(), and executes it through AtomicAttack.
• Updates "Defining your own" to point authors at the group modules.

Both the .py and executed .ipynb are updated and in sync.