Reorganize docs CI jobs and add separate i18n link checking

What does this MR do?

This MR reorganizes the documentation CI jobs to better separate standard documentation checks from internationalization (i18n) checks:

• Reorders jobs to group standard docs checks together, followed by i18n-specific checks
• Creates check_i18n_docs_links job specifically for i18n documentation link checking (manual, allowed to fail)
• Updates docs-i18n-lint paths job to only run in localization pipelines instead of all pipeline types

The changes improve the CI structure by making it clearer which jobs apply to standard documentation versus internationalized documentation, and ensures link checking runs appropriately for both.

Related issues

Related gitlab-com/localization/docs-site-localization#734

Related gitlab-com/localization/docs-site-localization#333 (closed)

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated - to be added in gitlab-org/gitlab!212317 (merged)
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart - gitlab-org/cloud-native/gitlab-operator!1319 (merged)