decree-docs: config file and template override system

[zeevdr]

Description

A decree-docs.yaml config file with a global section plus per-format sections, where every key has a CLI flag twin and flags win (terraform-docs model). Template customization: an export-templates command dumps the embedded built-in templates for editing, and a repeatable --template-files flag layers user templates where later defines override earlier ones (helm-docs model).

Acceptance criteria

• Config file and flag parity with documented precedence: flags > config file > defaults.
• export-templates writes the embedded template set to a directory; --template-files layering can override any named template.
• Test coverage: precedence unit tests and a golden test rendering with a user-overridden template.