docs: restore the pg_partman how-to heading my #1315 edit removed

[jdatcmd]

My mistake, now on main. In #1315's docs commit (3627327), I inserted the "Count distinct values and percentiles approximately" section by replacing the heading ## Manage time partitions with pg_partman instead of inserting before it. Since d208bce, the pg_partman steps sit headless under the approximate-aggregates section. Every docs check passed, because the text is still valid Markdown. I found it while rebasing #1311, whose section sits next to it.

The regression check first, in both twins:

• a docs_style.sh arm and an independent test_docs_how_to_sections.py find the section by its heading and require it to open with the pg_partman paragraph;
• red on main (d208bce), got 0, want 1, in both;
• the twin is cluster-free, so it's in NO_CLUSTER, with a TESTS.md section 95; guard_tests 414 → 415 by collection.

The fix: two lines in docs/how-to.md, the heading and its blank line.

Evidence: docs_style green; the twin passes; the guard half is 414 passed and 1 failed. The failure is test_build_refusal, which fails the same way on unmodified main here.

docs_style isn't a ledger suite, so no ledger rows are owed.

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[OffgridwithJD]

Approved at 61a08d3. Reproduced here:

• The heading: ## Manage time partitions with pg_partman appears 0 times in origin/main's docs/how-to.md and once in this PR's, followed by the pg_partman paragraph.
• test_docs_how_to_sections.py: passes on this PR. With origin/main's how-to.md copied in, it fails, so it's red for the right reason.

This matters for my #1317 too. Its 'Move cold partitions to Parquet' section sits right after the pg_partman steps, and on main those steps currently read as part of the approximate-aggregates section. I'll rebase onto this once it lands.

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