Support document variants with zero code

[dannyadair]

A document creating a single PDF does not need any coding.
When a document needs to create multiple PDFs for variants of itself, a bit of coding is currently still required:

If the document project contains a bake.py which defines process_document(), this will be called instead of the standard processing.

For Kiwi PyCon, we create "Material Specs" for sponsors in multiple variants, one for each sponsorship tier.
The project contains a bake.py:

"""Material specs document processor."""

from pdfbaker.document import PDFBakerDocument


def process_document(document: PDFBakerDocument) -> None:
    """Process and generate one material specs document for each tier."""
    document_name = document.name
    document_config = document.config.copy()

    for tier in document_config["tiers"]:
        document.baker.info("Processing tier %s", tier["name"])
        document.config = document_config.copy()
        document.config.update(
            {
                "tier": tier,
                "filename": f"{document_config['base_filename']} - {tier['name']}",
                "pages": tier["pages"],
                "sponsor_level": tier["sponsor_level"],
                "specs_items": tier.get("specs_items", []),
            }
        )
        document.name = f"{document_name}_{tier['name']}"  # Use original name
        document.process()

This can be generalised into pdfbaker itself to have variants with zero code.

Introduce a naming convention for a document's YAML configuration:

• "documents" list of documents, can be string or new object with "name" and (optionally) "variants"
• "variants" basically what we have as "sponsorship_tiers": they have their own "pages", "filename" and any other config
• variant config is merged with (already merged) document config for the template context - one document per variant

[dannyadair]

Implemented in dd17648

If a config has variants, a PDF will be created for each variant.
The value of filename is now rendered like a Jinja2 template and variant is the current variant config.

In our project we have

filename: "Your Brand at Kiwi PyCon - {{ variant.name }}"

and no more Python code required at all :)