Enforce docs-i18n-lint markdown + custom markdown lint rules

What does this MR do and why?

This MR implements proper markdown linting configuration for our translated documentation in the /doc-locale directory.

Key changes:

1. Adds custom .markdownlint-cli2.yaml configuration specifically for translated content
2. Updates CI linting scripts to use the locale-specific config instead of the root project config
3. Fixes path resolution issues that were causing the linter to use incorrect configuration files

Why are we doing this?

The current markdown linting was using the English-focused rules on translated content, causing false positives and making it difficult to identify true errors, such as extra spaces.

This MR is part of our broader effort to automate cleanup of target files and ensure proper validation of translated content.

References

1. Related https://gitlab.com/gitlab-com/localization/docs-site-localization/-/issues/183
2. Related https://gitlab.com/gitlab-com/localization/docs-site-localization/-/issues/269

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run the i18n lint script -> ./scripts/i18n_lint_doc.sh
2. Pull this branch down locally into your local docs site build
3. Run make view from the docs project
4. View /ja-jp/ URLs with changed content

MR acceptance checklist

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