crystal docs: use shard.yml version when no git tag present

[superhawk610]

When generating project documentation with crystal docs, the project version may be inferred from either the latest git tag or the version key in shard.yml. However, the version in shard.yml will only be used if the project is not a git repository. This requirement is counter-intuitive, and means that a project created with

$ crystal init app my_app

won't generate documentation out of the box without explicitly specifying --project-version:

$ crystal docs
Couldn't determine version from git or shard.yml, please provide --project-version option

This PR allows the version in shard.yml to be used regardless of whether or not a project is contained in a git repository; the latest git tag will still be used (if present), so I think this preserves the intention of the original implementation, while also improving the out-of-the-box DX for new projects.

[straight-shoota]

For reference, the original discussion of this feature was in #8792 and #4754.

The shard.yml fallback was added for the use case of non git-based distribution. But I suppose it should be good when not at a git tag as well.

[beta-ziliani]

Thanks @superhawk610 !