Documentation master task list

- [x] Improve Data.Sequence module description (#478)
- [x] Improve Data.Map (Lazy and Strict variants) module descriptions (#497)
- [x] Improve Data.IntMap (Lazy and Strict variants) module descriptions (#507)
- [x] Improve Data.Set module description (#502)
- [x] Improve Data.IntSet module description (#507)
- [x] Reorder Data.Map functions in Haddocks (#519)
- [x] Reorder Data.IntMap functions in Haddocks (#519)
- [x] Reorder Data.Set functions in Haddocks (#531)
- [x] Reorder Data.IntSet functions in Haddocks (#531)
- [ ] Improve Data.Graph documentation (#532)
- [ ] Add examples to Data.Graph Haddocks (#532)
- [x] Improve Data.Tree documentation (#534)
- [x] Add examples to Data.Tree Haddocks (#534)
- [ ] Document all necessary class instances (#475)
- [ ] Consistently format Haddock examples in expandable sections throughout package
- [ ] Gather consensus on Haddock format (#486) and use throughout package
- [ ] Add longer Set examples to bottom of [Set introduction](https://haskell-containers.readthedocs.io/en/latest/sets.html)
- [ ] Add longer Map examples to bottom of [Map introduction](https://haskell-containers.readthedocs.io/en/latest/maps.html)
- [ ] Add longer Sequence examples to bottom of [Sequence introduction](https://haskell-containers.readthedocs.io/en/latest/sequence.html)
- [ ] Add introduction for Data.Tree at https://haskell-containers.readthedocs.io
- [ ] Add introduction for Data.Graph at https://haskell-containers.readthedocs.io
- [ ] Document Data.Graph complexities (#157)

**Notes:**
- Module Haddock descriptions should:
+ link out to appropriate page at https://haskell-containers.readthedocs.io
+ follow the format of the [new Data.Sequence module description](https://hackage.haskell.org/package/containers-0.5.11.0/docs/Data-Sequence.html)
+ link to appropriate [performance benchmarks](https://github.com/haskell-perf)