fix(aggregate): support mean as synonym avg

[BaymaxHWY]

Which issue does this PR close?

Closes #2922

Rationale for this change

What changes are included in this PR?

1. update some error link in CONTRIBUTING.md
2. support mean aggregate

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #2923 (d69b1cf) into master (9401d6d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2923      +/-   ##
==========================================
+ Coverage   85.29%   85.31%   +0.01%     
==========================================
  Files         274      273       -1     
  Lines       49229    49287      +58     
==========================================
+ Hits        41991    42048      +57     
- Misses       7238     7239       +1     

Impacted Files	Coverage Δ
datafusion/core/tests/sql/aggregates.rs	99.28% <100.00%> (+0.01%)	⬆️
datafusion/expr/src/aggregate_function.rs	92.25% <100.00%> (+0.02%)	⬆️
datafusion/expr/src/binary_rule.rs	84.65% <0.00%> (-0.39%)	⬇️
datafusion/expr/src/logical_plan/builder.rs	89.64% <0.00%> (-0.22%)	⬇️
datafusion/optimizer/src/projection_push_down.rs	98.06% <0.00%> (-0.03%)	⬇️
datafusion/core/src/physical_plan/planner.rs	81.04% <0.00%> (-0.02%)	⬇️
datafusion/core/src/config.rs	92.50% <0.00%> (ø)
datafusion/core/src/scalar.rs
datafusion/physical-expr/src/from_slice.rs
datafusion/common/src/from_slice.rs	100.00% <0.00%> (ø)
... and 7 more

---

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[alamb]

Looks good to me; Thank you @BaymaxHWY

[andygrove]

Thanks @BaymaxHWY!

[ursabot]

Benchmark runs are scheduled for baseline = 36b82d7 and contender = 13f026c. 13f026c 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