[i18n & CI] Add default_lang_commit to front matter of all non en pages

[chalin]

• Contributes to [i18n] Propose way to associate translations to specific commit of default lang page #4582
• For each i18n page:
  • Adds the commit ref of the en page version from which the i18n page was translated from
• Updates scripts/i18n-check.sh:
  • Add -u flag used to update or add the default_lang_commit field value to i18n pages
  • Adds support for default target of content

Other than the script, all other files were updated by running the following command:

$ ./scripts/i18n-check.sh -u                           
1       1       content/en/docs/demo/_index.md - content/zh/docs/demo/_index.md
30      30      content/en/docs/concepts/components.md - content/zh/docs/concepts/components.md
1       1       content/en/docs/kubernetes/_index.md - content/zh/docs/kubernetes/_index.md
13      1       content/en/_index.md - content/zh/_index.md

Note: whenever a new i18n page is added or an existing page is updated, its default_lang_commit field value must be updated as well to match the main commit from which the i18n page addition/updates are based on.