[tlBMC/g3doc] Document Collectors Overview

Created an overview page for tlBMC collectors in `modules/collectors.md`.
Lists the different collector types (FRU, Sensor, Thermal, Metric, GPIO, Power Fault Log, SEL, PCIe) and provides comprehensive descriptions.

PiperOrigin-RevId: 932120869
Change-Id: I8d03709d866d3d0fda527acdef8dfda49b5bc97d

gbmcweb

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