Add amcheck to postgresql extension doc

What does this MR do and why?

This change adds a new PostgreSQL database extension called amcheck to GitLab's extensions list. The amcheck extension provides functions that help verify and check the integrity of database relations, essentially helping to detect and diagnose potential database corruption issues. The documentation is updated to show that this extension (version 1.3) should be installed alongside the existing required extensions like pg_trgm, btree_gist, and plpgsql.

References

• Add amcheck extensions (omnibus-gitlab!8589 - merged)
• Add amcheck extension (gitlab-org/charts/gitlab!4426 - merged)
• Related to #370622 (closed)

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