docs: give SQL one section with a page per construct

SELECT and Enterprise SQL sat apart in Query while documenting the same engine.
They are now one SQL group: overview, Select, Aggregation, Join, Window,
Sampling, Functions -- one page per construct, with the Enterprise client-setup
and FTS pages merged in rather than parallel to them.

Sampling is new. TABLESAMPLE is implemented and was undocumented: BERNOULLI
picks rows, SYSTEM picks Lance fragments, REPEATABLE fixes the draw so a sample
works as a stable evaluation set. Its refusals are documented too -- a
percentage over 100, a fractional row count, BUCKET, the SAMPLE spelling and
the OFFSET modifier are all errors, because a sampling clause that were read and
not applied would be a wrong answer rather than a missing feature.

Schema & Data Types is one page again, with the multimodal content merged into
its blob section rather than sitting beside it.

Agentic Skills stops being a tab. Both its pages are tutorials, so they join the
Tutorials tab; the Basics page that introduces them stays.

docs/web/quickstart.mdx

@@ -454,7 +454,7 @@ These snippets load the local image file and store the bytes in an `image` colum
     </CodeBlock>
 </CodeGroup >
 
-For more examples, see the [multimodal data](/tables/multimodal) section.
+For more examples, see [Data types](/basics/data-types#blob).
 
 ## 8\. Train on the result {#8-train}