doc: clarify libfuzzer-nosan preset uses build_fuzz_nosan dir #35274

The fuzzing quickstart documents the libfuzzer preset with build_fuzz as the build directory, then mentions libfuzzer-nosan as an alternative without noting that this preset uses a different binary directory (build_fuzz_nosan, per CMakePresets.json).

A reader following the quickstart with --preset=libfuzzer-nosan and then running cmake --build build_fuzz as shown will operate against the wrong (or empty) directory.

This PR:

• Adds a one-line note next to the first mention of the nosan preset in the Quickstart, pointing out the directory difference.
• Adds a small concrete code block in the "Run without sanitizers for increased throughput" section showing the corresponding build_fuzz_nosan commands.

Pure docs; no code changes.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35274.

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It's probably enough to mention it in a single place - concept ACK