About/Roadmap: Gutenberg demo URL should be translatable

[audrasjb]

Since Locales have a localized Gutenberg demo page, I think the link to the demo should be translatable as well.

Related code in page-about-roadmap.php:

<?php
printf(
        /* translators: %s: Link to Gutenberg demo page */
        wp_kses_post( __( 'WordPress is continually under development. Currently, work is underway on Phase 2 of the Gutenberg project. The Gutenberg project is a reimagination of the way we manage content on the web. Its goal is to broaden access to web presence, which is a foundation of successful modern businesses. Phase 1 was the new block editor, which was released in WordPress 5.0, you can <a href="%s">see that in action here</a>. In 2019 we’re focusing on Phase 2 which brings block editing to the entire site, not just posts and pages.', 'wporg' ) ),
        esc_url( 'https://wordpress.org/gutenberg/' )
);

[audrasjb]

Example:
demo page in fr_FR – ​https://fr.wordpress.org/gutenberg/
or de_DE – ​https://de.wordpress.org/gutenberg/

[SergeyBiryukov]

In 8119:

WordPress.org Main Theme: Roadmap: Link to Gutenberg demo page on the same domain.

Props audrasjb.
Fixes #4084.