This commit also has some other nice side-effects:
* The layout logic is unified for all page types, which should make it less surprising
* Page.Render now supports all types
* The legacy "indexes" type is removed from the template lookup order. This is an undocumented type from early Hugo days. This means that having a template in, say, `/layouts/indexes/list.html` will no longer work.
* The theme override logic is improved. As an example, an `index.html` in theme will now wn over a `_default/list.html` in the project, which most will expect.
Fixes #3005
Fixes #3245
This applies to both regular templates and shortcodes. So, if the site language is French and the output format is AMP, this is the (start) of the lookup order for the home page:
1. index.fr.amp.html
2. index.amp.html
3. index.fr.html
4. index.html
5. ...
Fixes #3360
This commit also adds a new command, docshelper, with some utility funcs that adds a JSON datafiles to /docs/data that would be a pain to create and maintain by hand.
Fixes #3242
