fix(docs): preserve receive policies links

[Osraka]

Summary

• Update the T6 blog links that still pointed at the old /docs/learn/tempo/receive-policies page.
• Add legacy redirects for /docs/learn/tempo/receive-policies and /learn/tempo/receive-policies to the current receive policies protocol page.
• Cover the new redirects in the existing docs routing test.

Test plan

• node -e "JSON.parse(require('fs').readFileSync('vercel.json','utf8')); console.log('vercel json ok')"
• git diff --check
• ./node_modules/.bin/biome check blogs/t6.md src/lib/docs-routing.test.ts vocs.config.ts vercel.json
• CI=true ./node_modules/.bin/vitest run src/lib/docs-routing.test.ts
• CI=true ./node_modules/.bin/tsgo --project tsconfig.json --noEmit

[vercel]

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