feat: customize getting started page (#5707)

* feat: add getting-started patch

This modifies the text on the Getting Started page to promote
coder/coder.

* feat: add --disable-getting-started-override

This adds a new CLI flag to code-server called
`--disable-getting-started` which will be used in Code to not use
Coder's custom Getting Started text.

* refactor: conditionally show coder getting started

This modifies the getting started patch changes to work with the new
`--disable-getting-started-override`.

The flag is false by default meaning the Coder getting started is shown.
By passing the flag to code-server, it will not be shown.

* docs: update faq for getting started override

* docs: update getting-started patch description

* fixup!: update patch

* fixup!: unit test

* feat: add more tests

* fixup!: use correct env var in tests

Co-authored-by: repo-ranger[bot] <39074581+repo-ranger[bot]@users.noreply.github.com>