i18n(Why GitLab): Implement /why-gitlab/ i18n & l10n

Changes

• Updated why gitlab template to expect localized content in contentful
• Enabled localized routes for why-gitlab

Production	Review app
https://about.gitlab.com/why-gitlab	https://3709-why-gitlab-i18n-l10n.about.gitlab-review.app/why-gitlab/

Step 2: Ensure that your changes comply with the following, where applicable:

• I, the Assignee, have run Axe tools on any updated pages, and fixed the relevant accessibility issues.
• These changes work on both Safari, Chrome, and Firefox.
• These changes have been reviewed for Visual Quality Assurance and Functional Quality Assurance on Mobile, Desktop, and Tablet.
• These changes work with our Google Analytics and SEO tools.
• These changes have been documented as expected.

Build Variables:

• Use Contentful Preview API

Closes #3709 (closed)