docs: document PRAGMA temp_store for VACUUM on large databases

[SAY-5]

Closes #430.

The maintainer wrote in #430 (comment):

It looks like PRAGMA temp_store was the right option to use here ... I'm going to turn this into a help-wanted documentation issue.

This adds a short paragraph to the existing Vacuum section in docs/python-api.rst describing the /tmp-fills-up symptom that triggered the issue and the PRAGMA temp_store = MEMORY workaround, with a runnable example and a link to the upstream SQLite PRAGMA temp_store docs.

Verified locally:

>>> from sqlite_utils import Database
>>> db = Database(memory=True)
>>> db.execute('PRAGMA temp_store = MEMORY')
>>> db.execute('PRAGMA temp_store').fetchone()
(2,)
>>> db.vacuum()

sphinx-build -n docs docs/_build produces no new warnings on the modified section (pre-existing reference.rst warnings are unchanged).

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📚 Documentation preview 📚: https://sqlite-utils--744.org.readthedocs.build/en/744/