docs: nest generated docs under docs/lib and docs/mcp

docs/lib/credentials/index.md

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+# Credentials
+
+::: mail_intake.credentials

docs/lib/credentials/pickle.md

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+# Pickle
+
+::: mail_intake.credentials.pickle

docs/lib/credentials/redis.md

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+# Redis
+
+::: mail_intake.credentials.redis

docs/lib/credentials/store.md

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+# Store
+
+::: mail_intake.credentials.store