feat(shortcodes): add caption argument to table shortcode

[LeonarddeR]

Adds a caption argument to the table shortcode that renders an HTML <caption> element, plus an opt-in caption-top boolean. Closes #1690.

Behavior

{{< table caption="Overview of records" >}}
| Name  | Type   |
|-------|--------|
| alpha | widget |
{{< /table >}}

The <caption> is injected as the table's last child, just before </table> — the same position simple-datatables re-appends it to, so a plain table and a data table share identical DOM. caption inherits the existing global string argument definition (comment overridden for the table context).

Position

caption-top defaults to false. Bottom is the native default of both Bootstrap and simple-datatables, so it's the least-surprising default; caption-top="true" adds Bootstrap's .caption-top class, moving the caption above the table via CSS caption-side — independent of DOM order.

PurgeCSS safelist

caption-top is emitted only under the opt-in, so on a site that never renders it the .caption-top{caption-side:top} rule would be purged from the compiled CSS, leaving the option silently inert. It's added to the standard safelist in both config/postcss.config.js and exampleSite/config/postcss.config.js — the same class of issue as the already-safelisted table-wrap / d-*-table-cell.

Verification

Built the example site and drove it in a browser. With no caption-top, the caption renders below the table (caption-side: bottom, last child); with caption-top="true" it flips to top (caption-side: top). Confirmed the .caption-top rule survives PurgeCSS after safelisting (it was purged, and the opt-in had no effect, before). Verified simple-datatables preserves the server-rendered <caption> on data tables (it captures CAPTION nodes on init and re-appends them on render). A caption fixture is added to the table-demo page. Existing table fixtures are unaffected; pnpm lint passes.

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