Enable link checking (paths) across languages

[chalin]

Scope

• Fix broken links
• Normalize file names, to match default locale (en)
• Normalize links, e.g., add trailing slashes

Check local paths for all languages

• bn
• de
• en - [CI & docs] Set up link checker + GH action + fix en links #3188
• es - [es] Enable link checking and normalize links #3223
• fr - [fr] Enable link checking, and normalize links, add missing #3197
• hi
• it
• ja - [ja] Enable link checking, and normalize links, add missing #3205
• ko
• pt-br - [pt] Enable link checking and normalize links #3256
• ru
• tr
• ur
• zh-cn
• zh-tw

Later

Once all local paths are being checked, we might consider enhancing the link checking to also include external links. As you can imagine that is more involved, so it's best handled separately.

/cc @cjyabraham @nate-double-u

[nate-double-u]

This is a good project to undertake, thanks for looking at it @chalin

[Okabe-Junya]

Hello folks!!

As one of the Japanese l10n teams/reviewers, let me work on the Japanese enablement :)