docs: port pg_clickhouse v0.3.2 updates from upstream

[mintlify]

Summary

Ports the upstream clickhouse-docs#6420 (Update pg_clickhouse docs to v0.3.2) to the Mintlify site.

Changes

Applied to both Mintlify mirrors of these pages:

• products/managed-postgres/extensions/pg_clickhouse/{introduction,reference}.mdx
• integrations/connectors/tools/pg_clickhouse/{introduction,reference}.mdx

introduction.mdx:

• Bumped pg_clickhouse Docker tag and postgresql-server package from 18 → 19.

reference.mdx:

• Added new CREATE SERVER options: compression, min_tls_version, secure.
• Added regexp_match → extractGroups / extractAll + arraySlice function mapping.
• Rewrote the regex-flags pushdown section: replaced the old bullet list with the new flag-translation table (i, m, n, p, s, t, w) and the newline-behavior comparison table.
• Updated the pushed-down example from (?i-s) → (?i).
• Added a note about regexp_match returning NULL vs. empty array and COALESCE() workaround.
• Updated linked Postgres docs URL from /docs/18/ → /docs/current/.

Context

• Slugs and file paths were preserved per the migration policy (no renames).
• Slug-only-relative link rewrites already present in the Mintlify copy (/reference/functions/... etc.) were preserved when porting the new regexp_match bullet, so internal links continue to resolve.
• Mintlify-specific frontmatter (sidebarTitle) and admonition syntax (<Tip>, <Note>, <Warning>) were left untouched — only the v0.3.2 content updates were ported.
• mint validate could not finish in the sandbox (timed out), so the PR has not been validated locally; changes are limited to content edits in 4 files with no structural or link-target changes beyond those listed above.

---

Note

Low Risk
Documentation-only edits in four MDX files with no application code or auth changes; risk is limited to possible doc inconsistencies (e.g. reference “Getting started” Docker snippets may still show :18 where introduction was bumped to :19).

Overview
Ports pg_clickhouse v0.3.2 documentation into the Mintlify site, mirrored in both integrations/connectors/tools/pg_clickhouse/ and products/managed-postgres/extensions/pg_clickhouse/.

Introduction pages bump the quick-start Docker image tag and APT package from PostgreSQL 18 → 19 (ghcr.io/clickhouse/pg_clickhouse:19, postgresql-server-19).

Reference pages document new CREATE SERVER options: compression (binary driver), min_tls_version, and secure (TLS auto/on/off with default ports). They add regexp_match pushdown mapping to ClickHouse extractGroups / extractAll + arraySlice, and expand the regex compatibility section with flag-translation and newline-behavior tables (example pushdown now uses (?i) instead of (?i-s)), plus guidance on regexp_match NULL vs empty array and COALESCE(). External Postgres doc links move from /docs/18/ to /docs/current/.

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