docs: new styling docs / advanced / reusable themes

[sissbruecker]

Adds the "Advanced / Creating Reusable Themes" article for the new styling docs.

Note

The PR targets a base branch for the new styling docs. That branch has the old styling docs removed and new articles are added gradually until the new section is ready. Cross-references between new articles will be filled in later, respective sections have been marked with TODOs.

[github-actions]

AI Language Review

For the file "page-title" under the Advanced styling section, the use of "Vaadin applications" is inconsistent throughout the document. The capitalization of the term should be checked and standardized, either always capitalized as 'Vaadin Applications' or lowercase as 'Vaadin applications' for consistency, unless it is a proper noun or title case usage within a heading.

In the section describing "If the theme uses NPM assets...", the note on package recommendations mentions that it is recommended to use specific packages for themes used in Vaadin Spring Boot applications. It should be clarified why these package recommendations are suitable, to improve understanding of their significance.

Lastly, ensure that any term-specific names, such as "JAR file," consistently use all capital letters when referring to specific formats or industry-standard terms.