Add all scalar SQL functions to user guide

[andygrove]

Which issue does this PR close?

Closes #3065

Rationale for this change

Adds all SQL scalar functions rather than just listing a few timestamp-related ones.

Many of the functions have no description or example usage yet, but at least we have a list of what is available and once this is merged it will be easier for everyone to submit PRs to improve the content.

What changes are included in this PR?

• New scalar-functrions page
• Deprecated datafusion-functions page

Are there any user-facing changes?

No

[andygrove]

@kmitchener fyi

[alamb]

Thank you @andygrove

[codecov-commenter]

Codecov Report

Merging #3090 (d601876) into master (0e0931d) will increase coverage by 0.08%.
The diff coverage is 91.31%.

❗ Current head d601876 differs from pull request most recent head 3f03224. Consider uploading reports for the commit 3f03224 to get more accurate results

@@            Coverage Diff             @@
##           master    #3090      +/-   ##
==========================================
+ Coverage   85.85%   85.93%   +0.08%     
==========================================
  Files         289      289              
  Lines       51890    52118     +228     
==========================================
+ Hits        44548    44790     +242     
+ Misses       7342     7328      -14     

Impacted Files	Coverage Δ
...afusion/core/src/physical_plan/file_format/avro.rs	0.00% <ø> (ø)
datafusion/expr/src/columnar_value.rs	100.00% <ø> (ø)
datafusion/proto/src/from_proto.rs	35.32% <0.00%> (-0.22%)	⬇️
datafusion/proto/src/to_proto.rs	52.94% <0.00%> (-0.10%)	⬇️
...atafusion/physical-expr/src/unicode_expressions.rs	77.45% <76.92%> (+4.35%)	⬆️
datafusion/sql/src/planner.rs	82.00% <77.77%> (+0.05%)	⬆️
datafusion/core/tests/sql/mod.rs	97.79% <80.00%> (-0.31%)	⬇️
...tafusion/physical-expr/src/datetime_expressions.rs	83.52% <92.30%> (+5.60%)	⬆️
.../core/src/physical_plan/file_format/file_stream.rs	90.98% <95.83%> (+0.50%)	⬆️
datafusion/core/src/physical_optimizer/pruning.rs	94.75% <100.00%> (+0.58%)	⬆️
... and 18 more

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[ursabot]

Benchmark runs are scheduled for baseline = 3eb55e9 and contender = 3579e0a. 3579e0a 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