Fix inconstent warnings.warn in repocard.py

[Wauplin]

Fix #1978 cc @osanseviero

Let's stick to the rule from the Python official documentation:

warnings.warn() in library code if the issue is avoidable and the client application should be modified to eliminate the warning
logging.warning() if there is nothing the client application can do about the situation, but the event should still be noted

=> those 2 warnings are clearly not user's fault (incorrect readme)

[HuggingFaceDocBuilderDev]

It looks like you've updated code related to model or dataset cards in this PR.

Some content is duplicated among the following files. Please make sure that everything stays consistent.

• src/.../repocard.py
• src/.../datasetcard_template.md
• src/.../modelcard_template.md
• modelcard.md (hub-docs repo)
• docs/hub/model-cards.md (hub-docs repo)
• docs/hub/model-card-annotated.md (hub-docs repo)
• datasetcard.md (hub-docs repo)
• docs/hub/datasets-cards.md (hub-docs repo)

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[osanseviero]

Thank you!