Generator extensions: user-defined output generators

[gennaroprota]

Splitting this out per the #1128 / #1196 discussion. Transformation extensions (scripts that mutate the corpus) shipped in #1196. Generator extensions are the separate capability: letting users add or customize an output generator without touching MrDocs core.

The concrete first approach is tracked in #1082 (format-agnostic Handlebars generator: a base generator parameterized by format, templates under share/mrdocs/addons/generator/<format>/, an escape map, supplemental addon directories; e.g. a Markdown generator). This issue is the umbrella for the capability.

Open questions:

• Template-only generators (the Format-agnostic Handlebars generator extension #1082 flavor) vs script-defined generators (interpreter hooks).
• How a custom format registers and how output file extensions map to template directories.
• Escaping rules per format.
• Interaction with addon layering / supplemental directories.

Distinct from #1210 (richer transformation entry points), #1213, and #1215, which are all about transformation extensions, not generators.