Refactor the Onionbalance documentation

Tasks

• Update and move some of the old v2 docs to the "main" docs.
• Update and move the v3 docs to the "main" docs.
• Remove the old v2 docs.
• Review.
• Apply related changes in the Ecosystem docs (handled by ecosystem!12 (merged)).

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)