MINOR: User Guide: Move expressions to top-level page

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Move expressions to top-level page to make them easier to find

What changes are included in this PR?

Move expressions to top-level page to make them easier to find

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #3134 (4b36e33) into master (b1765f7) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #3134 +/- ##
=======================================
Coverage 85.95% 85.95% =======================================
Files 291 291 Lines 52382 52382 =======================================
Hits 45025 45025 Misses 7357 7357 

Impacted Files	Coverage Δ
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	77.95% <0.00%> (+0.17%)	⬆️

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[ursabot]

Benchmark runs are scheduled for baseline = ff53f8b and contender = 6fe6dce. 6fe6dce 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