After the handbook was split into smaller pages by https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/4939 many of the old links needed updating

Updated applications page

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The Production build section was lost in the middle of the file,
between two sections that should have been grouped in the Contributing
section. It's now reordered properly.

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When omitted, it's inferred from the filename. It's only required when
setting a specific publication time, so we've added notes to the README.

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