Add documentation for updating / upgrading an existing installation of code-server

[wbobeirne]

We don't have anything in README.md, install.md, or FAQ.md on how this is done.

[snazzybunny]

I’d like this as well so we can preserve settings and extensions. Thanks!

[nhooyr]

@snazzybunny You just reinstall. Settings and extensions are stored separately and maintained between installs.

[nhooyr]

Done in one of the recent documentation update PRs. The release notes also mention how to upgrade now.