docs: note that replacement MVs must keep the same output schema

[claude]

Requested by Nate Stewart, Pranshu Maheshwari · Slack thread

Motivation

A customer hit the error replacement schema differs from target schema when
adding new output fields to a materialized view via CREATE REPLACEMENT MATERIALIZED VIEW. The constraint that a replacement must have the identical
output schema was documented only implicitly on the replace-materialized-view
page, so this makes it prominent.

Description

Before: Step 3 of the replace-materialized-view page did not call out that
the replacement must have the same output schema as the view it replaces.

After: Step 3 now includes a {{< note >}} stating that a replacement
materialized view must keep the same output schema (column names, types, order,
nullability, and keys), that schema changes are not supported including
non-breaking changes such as adding a new column, and that a mismatch fails with
the error replacement schema differs from target schema.

The note is added in the Step 3 numbered list and matches the page's existing
note style.

Verification

Docs-only change. Reviewed the rendered structure to confirm the two note
blocks are well-formed and the following numbered list item still renders
correctly.

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Generated by Claude Code

[kay-kim]

So ... we already mention in line 244 ... and I don't mind the repetition ... but is it then, that we just want to move up the first two bullet points up here (and include the must)?

[maheshwarip]

I intentionally added the repetition! But yes maybe just moving the bullet points up will help