doc: Add privacy.md consolidating network privacy recommendations #35858

Suggested by ismaelsadeeq in #29415.

Creates doc/privacy.md as a single home for Bitcoin Core's privacy tools for transaction broadcasting and network connectivity.

• Commit 1: Move the "Privacy recommendations" sections from doc/tor.md and doc/i2p.md into privacy.md, replacing them with pointers. The two files contained the same multi-network unlinkability warning duplicated verbatim; this consolidation removes the duplication. No content changes.

• Commit 2: Add a "Transaction broadcasting privacy" section documenting -privatebroadcast, its prerequisites (Tor or I2P), current scope (sendrawtransaction only, with a link to the tracking issue for planned wallet-level support), and the related RPCs (getprivatebroadcastinfo, abortprivatebroadcast).

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35858.

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