feat(Documentation/api): add some intro material

add some intro material and normalize the headers to the rest of the
coreos/docs.
chore(Documentation/api): remove some whitespace
feat(Documentation/api): add a statistics section
fix(Documentation/api): fixup the names in stats

This was fixed in #449
feat(Documentation/api): document the store statistics
Merge pull request #444 from philips/document-stats-api

Documentation: document the stats API