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

changed milestone to %Review Readiness Presentation

assigned to @leo-singer

assigned to @roberto.depietri and unassigned @leo-singer

changed the description

mentioned in merge request !129 (closed)

marked the checklist item Remove referece to COVID-19 pandemic. as completed

Related merge request: !129 (closed)

marked the checklist item LIGO/Virgo to LIGO/Virgo/KAGRA. as completed

marked the checklist item Update example notices. as completed

closed with commit ec710c36