[Docs-Change] Add reference for expression statements

[Luna-Klatzer]

Describe the change

Add the reference section for expression statements, which should contain the following items:

• Explanation of the term expression and how it is tied together with an expression statement.
• Explanation of the terminology of an expression statement (basically an expression that is also a statement/ends with a ;).
• Showing examples of how it is used and what type there are.

These should contain the standard items such as Syntax, Examples and Additional Notes if required. It should also contain simple language and descriptive language to explain it in detail, as it is the foundation of programming in Para-C, like in any other modern language.

Additional context

Section: https://para-c.readthedocs.io/en/latest/lang_ref/statements/expression_statement.html