Fix broken charts styling examples, add Flow API example

[HerbertsVaadin]

Solves:
#4782
#76 (sort of)

Changes:

• Rewrote CSS Styling example to work and match V24 approach.
• Created a new example about styling through Java API
• Renamed Tab to "Chart Styling" instead of "CSS Styling"
• Rewrote second CSS example, as it was not working.
• Included in the examples the changing of the series / line colors as well.

[github-actions]

AI Language Review

1. Changes in the "CSS Styling" section:

   • Minor grammar consistency: In the sentence "In this example Java API is used...", consider adding a comma after "example" for better readability. It should be: "In this example, the Java API is used..."

2. Changes in the "CSS Styling Example" section:

   • In the code comment for .themes/{mytheme}/components/vaadin-chart.css, consider adding a period at the end of the comment for consistency with other comments in the documentation.

3. References to images in "figure" sections and elsewhere should ensure there's clarity on where these images can be viewed if they're not included directly in the documentation file. Consider adding alt text descriptions for accessibility if not already done in the actual documentation system.

[peholmst]

This PR is targeted at Vaadin 24. Does it apply to Vaadin 25 as well?

[rolfsmeds]

This PR is targeted at Vaadin 24. Does it apply to Vaadin 25 as well?

Yes, it's equally applicable to V25. However, in the V25 docs, we need to mention that you need to enable shadow DOM style injection to use it. There is a note in the V25 Styling section about that: https://github.com/vaadin/docs/blob/main/articles/styling/index.adoc

[rolfsmeds]

More of a question than a request to change... otherwise looks good to me.
Would be great to port to V25 docs (where it also needs to be noted that component shadow DOM css styling needs to be enabled)

[rolfsmeds]

LGTM, except for that period :)