[FLINK-37239][docs] Add examples for unhex, btrim, translate, elt and percentile

[gustavodemorais]

What is the purpose of the change

We currently have no examples for some functions in the documentation. 5 examples of these are unhex, btrim, percentile, translate and ETL. This PR adds examples to these.

Upon merging, I'll create another ticket for the chinese translation.

Brief change log

• Added one example for each of the functions

Verifying this change

• Checked docs locally

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? no

[flinkbot]

CI report:

• 8f5a88d Azure: SUCCESS

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[snuyanzin]

Sorry, during the first iteration I didn't test the examples...

Now it took me almost half an hour to realize that this is based on a continuous distribution...
I think we need to think about other example here...
It returns same result for the case of 0.5 for both
however in case of discrete distribution

SELECT PERCENTILE(age, 0.25) FROM (VALUES (0), (25), (50), (100)) AS age

I would expect output 25.0 however it shows different result (since the function is for continuous)

It should be explicitly mentioned in the doc since in SQL Standard there are PERCENTILE_CONT and PERCENTILE_DISC.

[snuyanzin]

Returns the exact percentile value of expr at the specified percentage in a group.

looks like description is also incorrect since usually by group it is meant within a limited group (discrete)