refactor: update ComboBox examples to Aura theme

[web-padawan]

• Moved "Style variants" to Styling sub-page
• Removed align-right and small variants
• Changed border width property to 2px
• Replaced Lumo custom CSS properties
• Changed the image size to use 36px

Note: custom CSS properties tables reused by input fields pages should be updated in a separate PR.
IMO this could be done after we update individual field components to make reviewing easier.

[github-actions]

AI Language Review

• In ComboBoxPresentation.java, the comment suggests using a separate stylesheet for CSS. Consider rephrasing the note to say "We recommend placing CSS in a separate stylesheet and encapsulating the styling in a new component."

• In combo-box-presentation.ts, the comment after the tag // end::combobox[] mentions inline styles, similar to the Java file. Ensure consistency in the wording of recommendations across all files.

• In ComboBoxStyles.java, the theme variant ComboBoxVariant.AURA_HELPER_ABOVE_FIELD is used. Ensure that this variant is correctly defined and intended, as it deviates from ComboBoxVariant.LUMO_ variants typically used with Vaadin components.

• In combo-box-presentation.tsx and related React examples, ensure consistency in CSS property names for compatibility: use variables prefixed with --lumo- instead of --aura-, unless "aura" is defined elsewhere in the project.

There are inconsistent uses of CSS custom property names and style recommendations across the Java, TypeScript, and React examples that may require alignment and verification of correct usage.

[web-padawan]

UPD: if we agree to add align variants in vaadin/web-components#10551, I'll then update Flow variant enums.
Also the small variant can be probably implemented in Aura based on the internal discussion.

[web-padawan]

Closing this for now as we agreed to proceed with a different approach.