[fr] Move new observability-primer page under docs

[chalin]

Originally posted by @chalin in #5833 (comment):

@iguitton - ah, it's because the newly added page isn't under docs. The page was published at:

• https://deploy-preview-5833--opentelemetry.netlify.app/fr/concepts/observability-primer/

You'll need to move it to be under content/fr/docs, i.e., https://opentelemetry.io/fr/docs/concepts/observability-primer/.

[chalin]

@open-telemetry/docs-maintainers - for the record, we could have detected this via:

$ npm run check:i18n | grep fr/
File not found  content/en/concepts/observability-primer.md - content/fr/concepts/observability-primer.md - en was removed or renamed

Should we fail a build check when locale pages are no longer found? Or maybe have a PR check that would add a comment to that effect? I'd say not at this point, but it might be worth considering.