Add Russian translation of the accessibility best practices guide

[1t1sCooL]

Adds the Russian translation of the accessibility best practices guide (_articles/accessibility-best-practices-for-your-project.md), which was the only article still missing from the otherwise complete _articles/ru/ set.

Following docs/translations.md:

• front matter field names unchanged; title/description translated, lang: ru set, the untranslated field removed;
• class, order and image kept identical to the English article;
• all links kept pointing to their original targets.

Similar in spirit to #3723 (ja translation of the security guide); ru translations of this article don't appear in the open PR list (#3736 is zh-hans, #3724 is Tamil).

[1t1sCooL]

The tests failure is the known language-switcher issue that affects any first translations of this article: the switcher on the new page links to every locale's version of the article, most of which don't exist yet. The Tamil translation PR (#3724) fails the same check for the same reason, and #3736 includes a fix that filters the language switcher to translated articles. Once that lands, this PR should go green on rebase — happy to update if you'd prefer a different approach.