Update observing schedule timeline plot and associated text

Looking at the preview https://emfollow.docs.ligo.org/-/userguide/-/jobs/3271426/artifacts/html/capabilities.html, I see a quite old plot of the observing schedule timeline (October 2023). We should probably update it to match the March 14 version. The text below the plot says

Only LHO and LLO are currently operating, with BNS range of 140-165 Mpc. These two facilities will be offline for commissioning for about 2 months starting January 16, 2024.

which should probably be updated as well, at least removing "Only".