Add indexes section to configuration for auto creation of index pages

[yaph]

Auto create index pages, that don't appear in document index, i.e. need not be excluded in get_docs calls in templates.

Sample config in site.yaml:

indexes:
  - url: /posts/
    docs: /post/
    template: index.html
  - url: /blog/
    docs: /blog/
    template: index.html