docs: explain weight and the v0.2.16 weight cap in the FAQ

[tcharchian]

Summary

• Add FAQ entries for Weight (rewards, confirmation, pricing, selection) and the previous-epoch weight cap (cap_weight) introduced in v0.2.16.
• Make it explicit that governance, BLS, and PoC/cPoC validation voting use the cap, while rewards still use Weight; new/returning Hosts have cap_weight = 0 for one epoch.
• Align related FAQ answers and glossary terms so voting power is no longer treated as the same thing as Weight.

Test plan

• Confirm the new FAQ anchors render: What is weight? and What is the weight cap? (after deploy).
• Check the v0.2.16 note callouts on both questions.
• Skim glossary entries for Weight, Weight cap, and Voting Power for consistency with the FAQ.