Generate Markdown + Asciidoc for Tasks and Pipelines

[adambkaplan]

Feature request

Add ability for tkn to generate a Markdown representation of any Task, Pipeline, or group of such objects (ex: hub, bundle).

Use case

• Generate Markdown docs that can be rendered into HTML with a static site generator, such as hugo.
• Generate asciidoc docs that can be rendered to HTML by a supported platform.
• Auto-generate the READMEs for task/pipeline catalogs.

These would likely need to provide a default template that can be configured/modified by an end user. crd-ref-docs provides a similar mechanism.

UI Example

tkn bundle doc <bundle> [task/pipeline] [name] --output=markdown
tkn pipeline doc <name> -o markdown
tkn task doc <name> -o markdown

Note: there are some inconsistencies in command naming conventions for hub that make this a bit more challenging.