[QualGroup] Define membership rules, merge sections

[uwendi]

This patch updates the LLVM Qualification Group documentation to reflect current group structure and practices:

• Restructures the Group Composition section (previously "Participation" + "Contributors")
• Adds a formal member table with affiliations and community handles
• Introduces participation principles
• Documents membership criteria, nomination process, and review cycle
• Corrects references to meeting materials (wrong path)

These changes aim to improve clarity, inclusiveness, and alignment with similar working groups (e.g., LLVM Security Response Team).

[uwendi]

Superseded by #157804