docs: flow tql cte

[discord9]

What's Changed in this PR

This PR documents the new CREATE FLOW capability introduced in GreptimeDB PR greptimedb#7702: using a simple TQL CTE in flow definitions, for example WITH ... AS (TQL EVAL ...) SELECT * FROM ....

The English docs in this PR now cover:

• the new CREATE FLOW syntax shape for TQL CTEs;
• a practical flow example showing how to wrap TQL EVAL in a CTE;
• the intended restrictions of this feature, including that only the simplest single-TQL-CTE form is supported in flows.

Updated pages:

• docs/reference/sql/create.md
• docs/user-guide/flow-computation/continuous-aggregation.md
• docs/user-guide/query-data/cte.md

Notes:

• This PR currently updates the English docs only.
• Localized docs will be updated in a follow-up change after the English wording is finalized.

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

[fengjiachun]

@discord9 En part LGTM, Please add Ch part and also sync the document into v1.0.

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[fengjiachun]

LGTM