Refactor processing: Simple case should be zero-code

[dannyadair]

Even a single document like the Kiwi PyCon Sponsorship Prospectus currently has its own bake.py - this should be built-in functionality.
Python code should only be needed if you want to get complicated (like the "one PDF per sponsorship tier" for the Material Specs document)

• always inject full (merged) config into template context
• always inject page number into template context

[dannyadair]

Done in e29faf3 (major refactoring)

The "single document" case is now zero code.
Our code for a document with multiple variants now only needs 22 lines.
Will improve this further, can also be brought to zero.