docs: document --log-lines flag and real log line numbers

[raphaelvigee]

Summary\n\nDocuments the user-facing changes introduced in hephbuild/heph#114:\n\n- New --log-lines N flag on heph run (default 10) — controls how many trailing log lines appear in a failing target's diagnostic box.\n- Log box line numbers now reflect real positions in the full log (e.g. 91–100 for the last 10 lines of a 100-line log, not renumbered from 1).\n\n## Changes\n\nAdded a "Reading failure output" section to website/docs/guides/inspecting-builds.md, placed between "Listing what exists" and "Reproducing the environment" as it naturally fits the failure-debugging flow of that guide.\n\nNo sidebar changes needed — the new content is part of the existing page.\n\n> website/docs/reference/cli.md is auto-generated and was not edited.\n\n## What was not documented\n\nhephbuild/heph#115 (feat(remote-cache): name object on stream-read errors and retries) is a purely internal change — it improves tracing spans and error message clarity for remote cache stream failures. No user-facing behavior changed; no docs needed.\n

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