Make extension limitations easier to find in docs

[jsjoeio]

There's some confusion about why some extension aren't found when you search and that's because people don't always realize the extension marketplace is different than the one in VS Code.

See here: #4271 (comment)

[code-asher]

Huh weird, I thought we used to have a notice in the extension sidebar that explained the difference; maybe we lost it along the way.

[jsjoeio]

No, we do! This person suggested adding it to the docs as an additional place. See here. But I guess after the switch to Open VSX, we might not need this

[code-asher]

Ahhh gotcha! Yeah we might not need it but I suppose it cannot hurt to be extra clear code-server does not use the official marketplace.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no activity occurs in the next 5 days.

[jsjoeio]

Not stale