Companion tool to manage renderings of READMEs

[Hirevo]

This was listed as a "thing yet to do" item in the project's README, for quite some time already, and implementation work for it has started, so here is its proper tracking issue.

[Hirevo]

So, I've been thinking about how to implement that tool and the fact is that it will need access to the same configuration as the registry itself to interact with its crates and update their rendered READMEs.
So I've started work to expose some of Alexandrie's internals by splitting them into library crates, that this tool could hook into to perform its work (#46).