Add documentation for document type extension field

[rmartin16]

Describe the bug

When document types are configured for an app, the extension field is required but not documented.

For instance:

[tool.briefcase.app.podium.document_type.deck]
description = "Podium Slide Deck"
extension = "podium"
icon = "src/podium/resources/podium-deck"
url = 'https://beeware.org/project/projects/applications/podium/'

Steps to reproduce

See #1770 for example of what happens when extension isn't configured.

Expected behavior

The need and purpose of extension is documented.

The difference between this extension field and the extension specified in the name of the table isn't immediately clear...so, these differences may need to be understood to properly document this. Additionally, it's also possible one should be falling back to another in the code.

Screenshots

No response

Environment

• Operating System: Sonoma
• Python version: 3.12.2
• Software versions:
  • Briefcase: 0.3.18.dev279+gdbe11cf0

Logs

See #1770.

Additional context

No response

[freakboy3742]

For anyone looking into this: https://github.com/beeware/podium is probably the only usage of this feature in practice; it's entirely possible the explicit extension attribute is an oversight, and it should be using the toml tag as the extension.

[AfiMaameDufie]

Hi @freakboy3742 ,
I am looking into this now.