docs(cycles): restructure page for clarity and remove repetition

[marc0olo]

Summary

Five changes to remove redundancy and improve heading accuracy:

• Intro: drop inaccurate "a unit of compute" appositive; fix two em-dashes
• Drop "What are cycles?" heading: the intro already defines cycles; replace the section's two redundant opening sentences with a single lead-in "Cycles cover four resource categories:" and keep the bullet list in place
• Rename "What canisters pay for" → "Pricing": the categories were just listed above; this section covers rates and cost mechanics
• Developer responsibility: remove opening sentence "Canisters pay for their own resources, which comes with ongoing obligations:" — echoes the intro verbatim; the heading is self-explanatory
• Cost predictability: remove "Stable unit economics" bullet — restates the XDR peg already explained in the subsection above

Sync recommendation

hand-written