Update charts theming docs for V14

[alvarezguille]

Theming docs are still showing the P2 way of theming.

• [ ] Migrate to use css and CSSImport
• [ ] Make sure that branch config for vaadin-docs is correct
• [ ] Bonus points for updating demos that are using a custom theme

[DiegoCardoso]

PRs for vaadin-docs done:

• V10: https://github.com/vaadin/vaadin-docs/pull/177
• V11: https://github.com/vaadin/vaadin-docs/pull/178
• V12: https://github.com/vaadin/vaadin-docs/pull/179
• V13: https://github.com/vaadin/vaadin-docs/pull/180
• V14: https://github.com/vaadin/vaadin-docs/pull/181