[libc++][CI] Moves CI badge to main README. (#77247)

The current CI badge is currently in libc++ documentation. This does not
seem the right place:
- The typical location on GitHub is on the main README.
- The documentation is shipped as part of the release:
- This link does not work in off-line mode. Currently our documentation
works in off-line mode.
- The status in the release documentation does not reflect the status of
the shipped library. So users looking at it may see a red status and get
confused.

This moves the badge to the README.