x/website: add clear warning against checking go.work into VCS

[goto1134]

Since the original proposal strongly suggests against checking go.work into VCS:

go.work files should not be checked into version control repos containing modules so that the go.work file in a module does not end up overriding the configuration a user created themselves outside of the module. The go.work documentation should contain clear warnings about this.
https://go.googlesource.com/proposal/+/master/design/45713-workspace.md#rationale-the-file

Please add a clear warning against checking go.work into VCS to the documentation here: https://go.dev/ref/mod#go-work-file

This issue is inspired by the following Reddit discussion: https://www.reddit.com/r/golang/comments/15v8i06/why_gowork_file_should_not_be_committed/

[gopherbot]

Change https://go.dev/cl/521515 mentions this issue: _content/ref/mod.md: add clear warning against checking go.work into VCS

[seankhliao]

Duplicate of #53502