Make "Tips for Getting Started" ready for translations

[remibetin]

• Add comments for translators in frontmatter
• Use ACKNOWLEDGEMENTS placeholder in footer
• Add missing last_updated attribute in some pages
• Delete useless order and layout attributes in writing.md
• Use relative links when linked-to page is in WAI website.
• Fix "Accessibility principles" link in writing.md

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[remibetin]

@shawna-slh I have tested all those changes locally with a full website build.

[remibetin]

QA done: ✅