docs: improve docs, use php 8 level features

[Chris53897]

i adjusted doc to

• use the new php 8 level features
• make is easier to use fullcalendar for beginners with the fullBundle integration
• change header to => headerToolbar https://fullcalendar.io/docs/headerToolbar

As there are some BC changes i would suggest to make a new major version.
i have some more ideas, but i will see how many free time i have to work on this.
So maybe wait a bit for a new major release.