docs: clarify self-hosted local embedding limits

[anirudh5harma]

Summary

• add a Local Embeddings self-hosting docs page that explains the current local embedding limitation for multilingual deployments
• link the new page from self-hosting navigation, configuration, and quickstart
• add a docs-test guard so we do not accidentally document unsupported embedding-model env vars as live config

Tests

• PATH="$HOME/.bun/bin:$PATH" bun run test:self-hosting
• PATH="$HOME/.bun/bin:$PATH" bunx turbo run check-types --filter='@supermemory/ai-sdk' --filter='@supermemory/memory-graph'
• PATH="$HOME/.bun/bin:$PATH" bunx biome ci --changed --since=origin/main --no-errors-on-unmatched

Caveat

This PR does not implement the full multilingual embedding-profile/reindex fix for #1104 because the public repo and server-v0.0.3 tag do not include the self-hosted server embedding worker, pgvector schema, or binary build source. It ships the public docs/test guardrails for the issue and documents the production fix shape without claiming unsupported env vars already work.

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[anirudh5harma]

@claude please review this PR. Focus on whether the docs accurately describe the current self-hosted local embedding limitation without overclaiming unimplemented model-selection behavior, and whether the docs-test guard covers the important edge cases for issue #1104.