docs: update udf docs for udtf

[tshauck]

Which issue does this PR close?

Closes #8545

Rationale for this change

Updating the docs to match the exciting UDTF addition.

What changes are included in this PR?

Updates the UDF library doc, makes minor style update to the example

Are these changes tested?

Are there any user-facing changes?

Yes, public docs update.

[alamb]

Thank you @tshauck -- this is great. I think it might help to add a little more motiviation about about why UDTFs are so cool and what types of things you can do with them, but we can also do that as a follow on PR. This PR is a great step forward

🚀

[alamb]

Maybe we can add some other examples of things one could do, for example

parse_url('http://foo.com')

Or point at the parquet_metadata function in datafusion-cli and note that the output of the table function can be processed like the output of any other table.

For example

❯ select filename, row_group_id, row_group_num_rows, row_group_bytes, stats_min, stats_max from parquet_metadata('./benchmarks/data/hits.parquet') where  column_id = 17 limit 10;
+--------------------------------+--------------+--------------------+-----------------+-----------+-----------+
| filename                       | row_group_id | row_group_num_rows | row_group_bytes | stats_min | stats_max |
+--------------------------------+--------------+--------------------+-----------------+-----------+-----------+
| ./benchmarks/data/hits.parquet | 0            | 450560             | 188921521       | 0         | 73256     |
| ./benchmarks/data/hits.parquet | 1            | 612174             | 210338885       | 0         | 109827    |
| ./benchmarks/data/hits.parquet | 2            | 344064             | 161242466       | 0         | 122484    |
| ./benchmarks/data/hits.parquet | 3            | 606208             | 235549898       | 0         | 121073    |
| ./benchmarks/data/hits.parquet | 4            | 335872             | 137103898       | 0         | 108996    |
| ./benchmarks/data/hits.parquet | 5            | 311296             | 145453612       | 0         | 108996    |
| ./benchmarks/data/hits.parquet | 6            | 303104             | 138833963       | 0         | 108996    |
| ./benchmarks/data/hits.parquet | 7            | 303104             | 191140113       | 0         | 73256     |
| ./benchmarks/data/hits.parquet | 8            | 573440             | 208038598       | 0         | 95823     |
| ./benchmarks/data/hits.parquet | 9            | 344064             | 147838157       | 0         | 73256     |
+--------------------------------+--------------+--------------------+-----------------+-----------+-----------+

[tshauck]

Thanks for the feedback! I just pushed a090783 which expands a bit on why they're nice and adds the parquet metadata use-case since it shows why they're nice for interactive analysis.

[alamb]

Thanks again @tshauck