Ship with a CLI (or cargo extension) that ouputs the resulting documentation to stdout

[jraygauthier]

It would make it easier for people using the README.md as a source of truth approach (#![doc = include_str!("../README.md")]) to use this nice tool's output as part of an out of build process that injects it directly in their README.md within some span delimiters:

## Feature flags
<!-- feature-flags-start -->
<!-- feature-flags-end --> 

->

# Called manually by users.
$ just doc-readme  # Some simple awk script here that use the new CLI for injection.

->

## Feature flags
<!-- feature-flags-start -->
MyFeatureFlagDocExpandedHere
<!-- feature-flags-end --> 

In CI, one can run some simple $ just check-doc-readme to make sure the README.md file is up-to-date.