Split Russian and English documentation sites

[D19ital]

Summary

• publish Russian and English documentation as Hugo multihost outputs
• keep Russian pages on docs.ergohaven.xyz and move English pages to docs.eh.industries
• add cross-domain language links, canonical URLs, hreflang tags, and brand links
• generate static redirects for existing /en/ URLs, with a 404 fallback for unknown paths
• trigger the English Pages deployment after successful source builds
• update internal documentation links to their current paths and domains

Validation

• Hugo 0.147.7 production build: 46 RU pages and 46 EN pages
• 35 generated address-specific redirect pages
• no missing internal documentation targets
• no remaining source links to docs.ergohaven.xyz/en/
• workflow files pass actionlint

Rollout

Do not merge until docs.eh.industries points to the prepared English Pages site. Merging this PR activates redirects on the current documentation domain.