docs: add hover card / tooltip example to eventMouseEnter

[kenlyle2]

Closes fullcalendar/fullcalendar#5517

What this adds

A Hover Card / Tooltip (no external library) section to eventMouseEnter.md covering:

• Vanilla JS implementation using getBoundingClientRect() + position: fixed
• The 120 ms hide delay pattern that lets the cursor travel from the event pill to the card without it disappearing mid-transit
• Viewport collision detection (horizontal flip + vertical clamp)
• Abbreviated React example using useState + useRef for the same pattern

Why no library

Third-party tooltip libraries (Tooltip.js, Tippy.js) expect to own the DOM lifecycle of their anchor element. FullCalendar re-renders event elements on navigation, which breaks most library attachment strategies. The position: fixed + state approach sidesteps this entirely and has zero additional dependencies.

This pattern is running in production at PostGlider, a social-media scheduling app built on Next.js + FullCalendar v6, across both month and week views.

Related

• Supersedes the broken Tooltip.js demo referenced in Update tooltip demo in docs to use a current library fullcalendar#5517
• No new dependencies introduced