Revise documentation around Simulator and the docs family

[mcencini]

Summary

• align TorchSim with the documentation structure used by the other pulserver projects
• make Simulator the canonical user-facing sequence extension point
• explain the two complementary sequence paths: handlers for incoming Pulseq/MRD descriptions and layout() for offline construction
• make the Framework examples an explicit Course and the remaining sections Tours
• add an Explanations landing page and TL;DR summaries
• centralize publication-scale figure styling with transparent light/dark-safe rendering
• update stale repository/documentation links after the move to the pulserver organization
• keep the functional *_sim calls documented as convenience wrappers rather than a second extension API

The implementation/API behavior is intentionally unchanged in this PR. Functional-wrapper parity is handled separately.