Add .crystaldoc.yml config file for configuring docs generator

[nobodywasishere]

Add a .crystaldoc.yml file that lists config options for crystaldoc, namely what apt packages should be installed before executing, and what commands should be executed before building docs. This would allow GTK4 binding docs to be built

https://forum.crystal-lang.org/t/crystaldoc-info-hosted-shard-api-documentation/6018/5?u=nobodywasishere
https://hugopl.github.io/gtk4.cr/

[BlobCodes]

It would also be cool to have a way to canonicalize projects if a .crystaldoc.yml were added.

For example, I mostly host my projects on GitLab and often create GitHub mirrors for greater visibility.
In that case, there are two identical projects on crystaldoc.

Could also help with forks - if a project doesn't un-canonicalize in the .crystaldoc.yml, it's probably only meant for contributions and not very relevant.

[straight-shoota]

@BlobCodes shard.yml has a repository key for exatly this purposre. I don't think there's a need for duplication.

[straight-shoota]

I don't think generating docs should have any requirements except for the source code.

Generating code for documented APIs via macros seems like a very questionable endeavour. Especially if that means the APIs are platform specific. Then a central API documentation doesn't make much sense because it can only represent one version.

A possible workaround could be to have one or multiple shards that each contains the generated code for a specific platform and thus provides API docs for that platform.

The only relevant configuration that could be useful in some cases is a specific file as entry point for the doc gerneator.