Change "LIGO/Virgo" to "LIGO/Virgo/KAGRA" and report new "Timeline" of the observational plan.
KAGRA joined the collaboration and we need to update the documentation accordingly.
- LIGO/Virgo to LIGO/Virgo/KAGRA.
- Update example notices.
- Remove referece to COVID-19 pandemic.
- Update O4 and O5 timeline to the current plan