Add an example for the variant generator and specify FQCNs used

- Provide an explicit example, with details of options and variants generated.
- Improve descriptions.
- Add use statement for `VariantGenerator` to code block.
- Define FQCN for `VariantGeneratorInterface`
Correct variable name
Add basic documentation for the VariationBundle
Merge branch 'master' into variation-bundle

Conflicts:
	bundles/map.rst.inc
Update link text to VariationBundle docs
Remove duplication in map
Fix the payments and shipping api parts
Add new sections and more docs
Initialize chapter about channels
Merge pull request #231 from adamelso/variation-bundle

Add basic documentation for the VariationBundle
Merge pull request #230 from adamelso/patch-3

Add an example for the variant generator and specify FQCNs used
Add missing parameter to sylius_variant config and correct descriptions
Merge pull request #234 from adamelso/variation-bundle-config-fix

Add missing parameter to sylius_variant config and correct descriptions
Add promotions API doc
Merge branch 'master' of github.com:Sylius/Sylius-Docs