docs: move custom-field size variants to styling page

[web-padawan]

• Moved "Size variants" to the Styling page - there's only "small" variant but I preserved the title for consistency
• Slightly rephrased the text to mention "individual" vs "internal" as the latter is more often used for shadow DOM

[github-actions]

AI Language Review

The updated document for "Custom Field" and "Custom Field Styling" contains some changes that require attention:

1. Repetition of Sections: The section titled "Native Input Fields" and "Size Variants" appears to be repeated in both the main "Custom Field" document and the "Custom Field Styling" document. Consider consolidating these sections to avoid redundancy.

2. Use of Word "variant" in "Styling" Section: In the "Custom Field Styling" document, the term "variant" is used multiple times to describe styles. Consistency in terminology should be ensured. If "variant" is deemed appropriate, ensure it is used consistently throughout the other documentation as well.

3. Title and Heading Formatting: Ensure consistent formatting of titles and headings across all sections. For instance, in "Custom Field Styling," the title "Style Variants" should have a larger font or different style to distinguish it from sub-headings.

4. Example Usage: The example sections might benefit from more detailed descriptions of what they illustrate, helping readers understand the context better.

5. Clarification on Custom Implementations: In sections discussing custom implementations, like custom parsers and formatters, providing additional context or examples might be helpful for clarity.

Make these adjustments to improve clarity and cohesion in the document.

[sissbruecker]

Even if there's only one variant, should we still have a table? For now the purpose would mainly be to convey that the small variant only works with Lumo. Also consistency with other component pages maybe.

[sissbruecker]

Actually there might be more?

[web-padawan]

Good point, thanks. Added the table.