Set page titles from translation document

[bahmutov]

Currently, our markdown files have the title of the page in the metadata above it. For example "Why Cypress" page comes from why-cypress.md with the title

---
title: Why Cypress?
---

When we translate this page, the metadata is also translated, for example the Chinese translation has which is correctly displayed

---
title: 为什么选择Cypress?
---

But for untranslated pages, the English title is displayed. Here is the Japanese translation

Note that we actually have Japanese translation for this title in file themes/cypress/languages/ja.yml

sidebar:
  guides:
    overview: 概要
    why-cypress: なぜCypress?

We should pull these titles for untranslated documents and use them instead of the metadata (if there is no metadata)

[jennifer-shehane]

Part of #1686