x/website: add a Go GC guide

[mknyszek]

As part of #48409 I proposed adding a new guide for the Go GC. This is an issue to track that work.

[gopherbot]

Change https://go.dev/cl/411954 mentions this issue: _content/doc: add work-in-progress gc-guide.html

[mknyszek]

Note for release folks: I marked this as a release blocker because we have outstanding links to this guide and work-in-progress page that mentions expected availability around the time of the 1.19 release.

However, this is not a hard blocker. If need be, we can drop this and update the work-in-progress page with a new expectation. I fully intend to have it ready before then, but if for some reason that becomes impossible, this issue is still useful for tracking the work to either update that work-in-progress page, or delete the links (from the SetMemoryLimit documentation, added in Go 1.19, and the Go 1.19 release notes).

[gopherbot]

Change https://go.dev/cl/414397 mentions this issue: _content/doc: add GC guide for Go 1.19