Build html docs in single plugin

[abelsromero]

• Fix html-plain generating 0 bytes output removing unused doctype
• Unify all html in single plugin config: each file uses its own execution with shared and custom attributes
• Set resources to avoid empty folders & place images only where necessary

The final result is that now we can get the docs in the desired structure for publishing with a single build. With the caveat of the interleaved html directory, but that could be removed easily now, even passed as command argument during CI.

From this point, it should not be hard to get a GH-action that pushes the docs on release 🎉

[vladimir-bukhtoyarov]

Thank you very much. Has been merged to master.