docs: drop the DataFrame API from the SQL manual

DataFrame support is not going ahead, so the pages documenting it come out:
the relational operations and the expression system, both of which described
only that API.

The manual was framed around having two query surfaces and choosing between
them. With one, that framing is gone — the overview now leads with SQL over
Arrow Flight SQL and what the dialect is, rather than with a comparison to
something that will not exist.

The function reference keeps everything that was about SQL: the DataFusion
version, how Lance columns map onto its type system, the pseudo-columns, blob
handling, and `fts()`. Only the section on reaching functions through the
Python wrapper is replaced, since the wrapper is going too.

Mentions of pandas and polars DataFrames elsewhere in the documentation are
unrelated and untouched.

docs/web/docs.json

@@ -150,11 +150,9 @@
             ]
           },
           {
-            "group": "SQL and DataFrame",
+            "group": "SQL",
             "pages": [
               "sql/index",
-              "sql/dataframe",
-              "sql/expressions",
               "sql/functions"
             ]
           },