User Guide: Add documentation for subquery syntax

[andygrove]

Which issue does this PR close?

Part of #3091

Rationale for this change

We should tell users that we support subqueries and how to use them.

What changes are included in this PR?

New section on subquery support.

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #3132 (627523b) into master (b1765f7) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #3132 +/- ##
==========================================
- Coverage 85.95% 85.87% -0.08% 
==========================================
Files 291 291 Lines 52382 52758 +376 ==========================================
+ Hits 45025 45307 +282 - Misses 7357 7451 +94 

Impacted Files	Coverage Δ
...on/physical-expr/src/expressions/binary/kernels.rs	52.17% <0.00%> (-10.99%)	⬇️
...ore/src/physical_plan/file_format/chunked_store.rs	62.26% <0.00%> (-5.09%)	⬇️
...tafusion/physical-expr/src/expressions/datetime.rs	84.22% <0.00%> (-2.58%)	⬇️
datafusion/core/tests/path_partition.rs	84.92% <0.00%> (-0.87%)	⬇️
datafusion/core/tests/sql/mod.rs	96.94% <0.00%> (-0.85%)	⬇️
datafusion/expr/src/window_frame.rs	92.43% <0.00%> (-0.85%)	⬇️
...sion/core/src/physical_plan/file_format/parquet.rs	94.56% <0.00%> (-0.79%)	⬇️
...afusion/core/src/datasource/file_format/parquet.rs	85.57% <0.00%> (-0.70%)	⬇️
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/common/src/scalar.rs	84.61% <0.00%> (-0.28%)	⬇️
... and 8 more

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[alamb]

Thanks @andygrove -- looking very nice 👌

[ursabot]

Benchmark runs are scheduled for baseline = 5778ef2 and contender = 4876cd6. 4876cd6 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java