Reorganize dataset section in user guide

I think the dataset section in user guide (http://scikit-learn.org/dev/datasets/index.html) is hard to follow. See the subsections:

5.1. General dataset API
5.2. Toy datasets
5.3. Sample images
5.4. Sample generators
5.5. Datasets in svmlight / libsvm format
5.6. Loading from external datasets
5.7. The Olivetti faces dataset
5.8. The 20 newsgroups text dataset
5.9. Downloading datasets from the mldata.org repository
5.10. The Labeled Faces in the Wild face recognition dataset
5.11. Forest covertypes
5.12. RCV1 dataset
5.13. Kddcup 99 dataset
5.14. Boston House Prices dataset
5.15. Breast Cancer Wisconsin (Diagnostic) Database
5.16. Diabetes dataset
5.17. Optical Recognition of Handwritten Digits Data Set
5.18. Iris Plants Database
5.19. Linnerrud dataset

I can’t find the logic behind such organization. I think we should first divide the section into two subsections: one for dataset loaders, one for sample generators. For dataset loaders, we can further divide according to fetch_ and load_. For sample generators, we can further divide according to different tasks (e.g., regression, classification). Related to #10555 What do others think?

[jnothman]

Whereas I think most users don’t care about a download that happens once. Therefore the main thing that characterises built-in datasets is their being small!​

[qinhanmin2014]

Closing since we’ve finished reorganizing the section in #11180 I’ve marked #11319 (which contains some further work on it) as 0.20 to attract reviewers.