[Subtask] Add user doc for column default value

[mchades]

Describe the subtask

Add user doc for column default value

Parent issue

#896

[hiirrxnn]

Please tell me precisely what is to be done here and I will get started on it .

[mchades]

Please tell me precisely what is to be done here and I will get started on it .

Thank you for volunteering!

The document with column default value depends on the expression document, because the default values of the column are represented by the expression. Would you like to document the expression first?

[hiirrxnn]

Could you please guide me on how to document the expression ?

[mchades]

Of course!
But this may require some prior knowledge. Are you familiar with Gravitino Expression or SQL Expression? If you are already familiar with those, we need a document named gravitino-expression-system.md. In this document, present the definition, implementation, and invocation method of the com.datastrato.gravitino.rel.expressions.Expression interface from the user's perspective. Provide corresponding REST and Java use cases, as well as equivalent SQL semantics. Finally, add a document link to any mentions of Expression in other documents.

I understand that this may be challenging for non-interface developers. If you feel the same way, I still have more accessible issues to choose from, such as #1640

[hiirrxnn]

Since I have no prior experience in Gravitino/SQL expression , I feel I am better suited for #1640 . Also , please let me know which topics/tech-stacks to be studied so I can solve a problem like this one , in the future.

[mchades]

Since I have no prior experience in Gravitino/SQL expression , I feel I am better suited for #1640 . Also , please let me know which topics/tech-stacks to be studied so I can solve a problem like this one , in the future.

Any one of the Gravitino catalog engines is good for study, such as Iceberg, Hive, MySQL, PostgreSQL