Use `Documentation` as a default `--config` option setting

In #421 I tried to adjust guides and render-guides so that it would use --config=Documentation as a default.

Since this idea was part of #421 where it didn't belong, and I failed with my approach, we may address this at a later time.

Description:

render-guides/packages/guides-cli/bin/guides does a very early interpretation of the "config" attribute:
https://github.com/phpDocumentor/guides/blob/main/packages/guides-cli/bin/guides#L54

That means, I'd have to inject fetching a default configuration (based on the path that is to be rendered) at this place.

However, the directory to render is calculated much later on (guides defines it as "docs", render-guides defines it as "Documentation" in a decorator).

So:

• In the guides binary, I can influence additional loading of a guides.xml - but I cannot get the default "Documentation" render directory, because that is performed AFTER the guides.xml loading.
• In our render-guides Decorator I can get the proper render directory, but I can no longer inject a guides.xml file (because the container is already built with the XML specifications as part of guides).

I could fetch a default "config" option in the case that a Directory is specified:

vendor/bin/guides Documentation

but I could not get the "right" directory name ("Documentation/") if I would call the binary just via:

vendor/bin/guides

So I think we'll need to retain '--config' as a mandatory argument.

For overview, current behavior:

vendor/bin/guides
# -> Renders "/Documentation", loads "/guides.xml"

vendor/bin/guides Documentation
# -> Renders "/Documentation", loads "/guides.xml"

vendor/bin/guides --config=Documentation
# -> Renders "/Documentation", loads "/Documentation/guides.xml" and "/guides.xml"

vendor/bin/guides --config=Documentation Documentation
# -> Renders "/Documentation", loads "/Documentation/guides.xml" and "/guides.xml"

My goal would be to make all four commands do the same: Render "Documentation", load "Documentation/guides.xml" and load "/guides.xml".

Start with render-guides/packages/guides-cli/bin/guides, especially its early config handling, then inspect the render-guides Decorator where the default render directory is selected. Compare all four documented command forms and define done as each rendering Documentation while loading both Documentation/guides.xml and the root guides.xml.