[FEATURE] Provide better documentation for new PPL users

[dai-chen]

Is your feature request related to a problem?
Currently there is user manual for PPL command and function. As more and more commands added, however, it may take time for a new user to figure out how to write a PPL query, especially those who have used SQL for years.

What solution would you like?
Add documentation for new PPL user to learn and migrate like what SPL provides: https://docs.splunk.com/Documentation/Splunk/latest/SearchReference/SQLtoSplunk. This doc can present how different, user-friendly and enhanced compared with SQL.

What alternatives have you considered?
Alternatively, we may provide an extra _plugins/_sql/translate?to=ppl API to translate simple SQL query to corresponding PPL query. This may be done by a visitor which visits common SQL operator and convert it to PPL command.

Do you have any additional context?
N/A