Localisation and internationalisation on several places

[zzap]

Issue Description

Plugins and Themes handbooks have took their relevant info from the old Codex page and there is a new Common APIs section for localisation and translation. And still we have this Codex page alive. This should be reviewed and move all the relevant info into Common APIs handbook.

URL of the Page with the Issue

• Codex: https://codex.wordpress.org/I18n_for_WordPress_Developers
• Themes - classic: https://developer.wordpress.org/themes/functionality/internationalization/
• Themes - block: https://developer.wordpress.org/themes/block-themes/internationalization/
• Plugins: https://developer.wordpress.org/plugins/internationalization/
• Common APIs: https://developer.wordpress.org/apis/handbook/internationalization/

Why is this a problem?

Because this info should live in one place. It's the same for all WordPress developers.

Suggested Fix

• [ ] Review all the info we currently have.
• [ ] Keep content that is up to date, remove outdated and add new if applicable.
• [ ] Update pages in Common APIs.
• [ ] Retire Codex page - by removing complete content and adding link to a Common APIs page(s).
• [ ] Remove duplicate content from Plugins handbook (as it is more common than plugin specific) and link to Common APIs page - cc @theMikeD

[github-actions[bot]]

Heads up @zzap - the "apis" label was applied to this issue.