i18n(ru): Primary pages were translated to russian

[Lootjs]

What kind of changes does this PR include?

• Changes or translations of Starlight docs site content

Description

I would like to share my translation of some documentation pages into Russian.

• Also, I've added Russian to the language selection block.
• In addition, I've corrected some inaccurate translations into Russian in the UI.

Could someone add the hacktoberfest-accepted label? 👀

p.s here is my previous PR to Astro Docs

[changeset-bot]

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Latest commit: 330607b

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[Lootjs]

Is it necessary to add changesets? 🤔

[delucis]

Thank you @Lootjs! I can see one small change needed here, otherwise looks great 🙌

[delucis]

Thank you! Ready to launch 🚀

[delucis]

Oh, actually, just realised one more thing to check: links to docs.astro.build can also use the /ru/ link instead of /en/. Will update these before merging.