L10 strings for static and instance methods and properties

[wbamberg]

This is the l10n companion to mdn/yari#7335, and adds the localised strings for static/instance methods/properties en-US, fr, es, zh-CN, so they can be shown in the sidebar. Other languages can be added in follow-up PRs.

To test:

• check out Yari
• check out this branch
• point Yari's CONTENT_ROOT at this branch
• visit some pages under Web/API in various languages:
  • en-US, fr, es, zh-CN should all show translated strings for "Static methods", "Static properties", "Instance methods", "Instance properties" in the sidebar
  • any other locales (e.g. ja, pt-BR) should show en-US strings

(like this: mdn/yari#7335 (comment))

This PR replaces #21690, which rotted.