enhance(macros/PreviousMenuNext): use the actual title of the document by default

[yin1999]

Summary

Fixes: #10274

Problem

We used to use localized slugs, so we could generate localized titles through the slugs. But now we have used unified slugs across locales, which causes the {{PreviousMenuNext}} macro to be unable to display localized titles for the menu link.

Solution

Solve this problem by retrieving the title of the page, if the page is not found:

• for translated documents: try to retrive the title from the default locale (en-US), if still not existed, fallback to generate it from the given slug.
• for en-US: fallback to generate it from the given slug.

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Screenshots

path	Before	After
/zh-CN/docs/Learn/Accessibility/CSS_and_JavaScript
/zh-CN/docs/Learn/Accessibility/HTML
/fr/docs/Learn/Accessibility/HTML

PreviousMenu macro

{{PreviousMenu}} macro reuses the {{PreviousMenuNext}} macro.

path	Before	After
/fr/docs/Learn/Accessibility/Accessibility_troubleshooting
/ja/docs/Learn/Accessibility/Accessibility_troubleshooting

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How did you test this change?

Run yarn dev and check the above pages.

[yin1999]

/cc @awxiaoxian2020 You may be interested in this :)

[LeoMcA]

Very nice, adds functionality and cleans up some code, a few small queries:

[LeoMcA]

Thanks for those answers, looks good with no changes!