DOC: re-organise docs for `instruments`

[attack68]

• [x] IRS (+Swap aliased)
  • [ ] Link spread examples elsewhere
• [x] SBS
• [x] FRA
• [ ] ZCS
• [ ] XCS
• [ ] FixedFloatXCS
• [ ] FixedFixedXCS
• [ ] FloatFixedXCS
• [ ] FXSwap
• [ ] NonMtmXCS
• [ ] NonMtmFixedFloatXCS
• [ ] NonMtmFixedFixedXCS
• [ ] FXSwap
• [x] FixedRateBond
• [x] Bill
• [x] BondFuture