docs: add bilingual DLC mods overview

[nowaytofindavailableone]

Summary

• Add a Russian DLC-mod overview and a mirrored English overview.
• Document the official DLC/regular-mod distinction, packaging routes, mixed-project bundles, .reddlc placement, and quest-DLC caveats.
• Add mirrored RU/EN navigation entries and the required AI-generated-content warnings.

Sources

• CDPR REDkit DLC Mods documentation: https://cdprojektred.atlassian.net/wiki/spaces/W3REDkit/pages/6328346
• CDPR REDkit quest how-to: https://cdprojektred.atlassian.net/wiki/spaces/W3REDkit/pages/6327243
• CDPR REDkit general information: https://cdprojektred.atlassian.net/wiki/spaces/W3REDkit/pages/6324268
• Detailed community pages are linked with canonical URLs from redkit-resources/docs.

Review gates

• Pass A retry A2: independent fact and translation reviewers both PASS.
• Pass B B1: both independent reviewers PASSed with MINOR findings; B1 was superseded after coordinator fixes.
• Pass B retry B2: independent fact and translation reviewers both PASS; no findings.

Validation

• Exact pinned base fails strict RU and EN MkDocs builds before patch application because of the pre-existing nav schema warning (Expected nav to be a list).
• Patched RU and EN CI-equivalent non-strict MkDocs builds: passed (274 files each).
• Static/local link checks: passed; live worktree remained unchanged during isolated checks.
• REDkit runtime and test-game execution: not run.