Migrate documentation for aggregate functions from aggregate_functions.md to code

[alamb]

Is your feature request related to a problem or challenge?

As part of #12740 this ticket is to migrate all the existing documentation for aggregate functions from the static file to code and to remove them from the static file.

Here is the current static content:

• doc source: https://github.com/apache/datafusion/blob/main/docs/source/user-guide/sql/aggregate_functions.md
• rendered: https://datafusion.apache.org/user-guide/sql/aggregate_functions.html
• Code source: https://github.com/apache/datafusion/tree/main/datafusion/functions-aggregate/src

Need Functions:

• Port / Add Documentation for ApproxDistinct
• Port / Add Documentation for ApproxMedian
• Port / Add Documentation for ApproxPercentileCont
• Port / Add Documentation for ApproxPercentileContWithWeight
• Port / Add Documentation for ArrayAgg
• Port / Add Documentation for Avg
• Port / Add Documentation for BoolAnd
• Port / Add Documentation for BoolOr
• Port / Add Documentation for Correlation
• Port / Add Documentation for Count
• Port / Add Documentation for CovarianceSample
• Port / Add Documentation for CovariancePopulation
• Port / Add Documentation for FirstValue
• Port / Add Documentation for LastValue
• Port / Add Documentation for Grouping
• Port / Add Documentation for Median
• Port / Add Documentation for Max
• Port / Add Documentation for Min
• Port / Add Documentation for NthValueAgg
• Port / Add Documentation for Regr
• Port / Add Documentation for Stddev
• Port / Add Documentation for StddevPop
• Port / Add Documentation for StringAgg
• Port / Add Documentation for Sum

Describe the solution you'd like

One (or more) PRs that

1. port the documentation for the above named functions from the static docs to code
2. Bonus points: add sql examples should be added for all functions
3. Remove the documentation from the static docs

Here are some example PRs of doing this:

1. Migrate documentation for all string functions from scalar_functions.md to code  #12775 from @Omega359
2. Port / Add Documentation for VarianceSample and VariancePopulation #12742

Describe alternatives you've considered

No response

Additional context

No response

[jonathanc-n]

take

[jonathanc-n]

Will work on this after @Omega359 approves of my first pr!