Docs: clarify selecting fields for included relations

[amyavari]

Based on Issue #959 and some other misunderstandings included in the docs, I tried to make it clearer for users to understand the deeper behavior of Spatie Query Builder when selecting fields for relationships.

• Updated the Installation & setup page with all available configuration options from the package config file.
• Added a note about default name convention for the Selecting fields for included relations section in the Selecting fields page, and fixed its example.
• Updated the example in the explanation for the convert_relation_names_to_snake_case_plural option.
• Updated the explanation for the convert_relation_table_name_strategy option to clarify its underlying behavior and purpose.
• Fixed outdated references in the Selecting included fields section on the Including relationships page.

[freekmurze]

Thanks!