docs: explain temporary storage for large database operations

[roli-lpci]

Closes #430.

Adds a focused troubleshooting section beside Database.vacuum() for large operations that fail with database or disk is full even when the database filesystem has room.

The section shows setting PRAGMA temp_store = MEMORY on the same connection when sufficient RAM is available, and selecting a writable temporary directory with SQLITE_TMPDIR before starting the CLI on Unix-like systems. It includes the relevant limits: VACUUM disk-space requirements, compile-time overrides, existing temporary objects being deleted when changing the pragma, and journal/WAL storage. It links SQLite's authoritative documentation and avoids the deprecated temp_store_directory pragma.

Validation:

• Executed the Python example and the CLI command on a disposable database. Both preserved all 20 test rows and returned PRAGMA integrity_check = ok; the requested temp_store value read back as 2.
• uv run --python 3.11 --group docs make -C docs html: build succeeded.
• Codespell on the modified page and git diff --check: passed.
• The original large-database disk-full failure was not reproduced, and this does not claim memory storage eliminates all disk requirements.

Prepared with AI assistance.

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📚 Documentation preview 📚: https://sqlite-utils--853.org.readthedocs.build/en/853/