Migrate documentation for all encoding functions from scalar_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 the above named functions from the static documentation 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/scalar_functions.md
• rendered: https://datafusion.apache.org/user-guide/sql/scalar_functions.html
• Code source: https://github.com/apache/datafusion/tree/main/datafusion/functions/src/encoding

Need Functions:
(see source)

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

[alamb]

Actually this is already done in #12668