User Guide: Add documentation for JOIN syntax

[andygrove]

Which issue does this PR close?

Part of #3091

Rationale for this change

Did you know that DataFusion supports joins? Possibly not if you are learning from the SQL reference in the user guide.

What changes are included in this PR?

Add documentation for JOIN syntax, with examples.

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #3130 (b7ac353) into master (b1765f7) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3130      +/-   ##
==========================================
- Coverage   85.95%   85.95%   -0.01%     
==========================================
  Files         291      291              
  Lines       52382    52382              
==========================================
- Hits        45025    45024       -1     
- Misses       7357     7358       +1     

Impacted Files	Coverage Δ
datafusion/expr/src/window_frame.rs	92.43% <0.00%> (-0.85%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	77.77% <0.00%> (ø)

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[ursabot]

Benchmark runs are scheduled for baseline = 15a9a4b and contender = 4cf0cf0. 4cf0cf0 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