Add skeleton of sql item (#1004)

* Add skeleton of sql item

Pretty sure an entry for sql is needed, but not sure how it should read

* Update markdown.md

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Co-authored-by: Mike Bostock <[email protected]>

Author: naltmann

docs/markdown.md

@@ -20,6 +20,7 @@ The front matter supports the following options:
 - **index** — whether to index this page if [search](./search) is enabled; defaults to true for listed pages
 - **keywords** <a href="https://github.com/observablehq/framework/releases/tag/v1.1.0" target="_blank" class="observablehq-version-badge" data-version="1.1.0" title="Added in v1.1.0"></a> - additional words to index for [search](./search); boosted at the same weight as the title
 - **draft** <a href="https://github.com/observablehq/framework/releases/tag/v1.1.0" target="_blank" class="observablehq-version-badge" data-version="1.1.0" title="Added in v1.1.0"></a> — whether to skip this page during build; drafts are also not listed in the default sidebar
+- - **sql** <a href="https://github.com/observablehq/framework/releases/tag/v1.2.0" target="_blank" class="observablehq-version-badge" data-version="1.2.0" title="Added in v1.2.0"></a> — table definitions for [SQL code blocks](./sql)
 
 ## Headings