Update docs for extensions change from OpenVSX to P3M

[cindyytong]

context

As a followup to this issue #12875, we should update our documentation

AC

1. Mention that P3M is used as default
   https://positron.posit.co/extensions.html#open-vsx

2. update references to open vsx to posit public package manager

[cindyytong]

@timtmok @juliasilge @jthomasmock anything missing in AC you want updated?

[juliasilge]

I already did this in posit-dev/positron-website#351.