[trivial] add more test examples

spec/examples/complex.schema.md

spec/examples/constants.schema.md

spec/examples/custom.schema.md

spec/examples/deepextending.schema.md

spec/examples/definitions.schema.md

spec/examples/enums.schema.md

spec/examples/example.schema.md

spec/examples/examples.schema.md

spec/examples/extending.schema.md

spec/examples/extensible.schema.md

spec/examples/simple.schema.md

spec/examples/simpletypes.schema.md