_content/doc/modules/pruning: add a placeholder redirecting to “Manag…

…ing dependencies”

I plan to add a more in-depth troubleshooting guide at this URL.
For now, I just need to URL to work so that I can point to it in
the 'go' command in Go 1.17 (in CL 332573).

For golang/go#36460

Change-Id: I95f03f76519dfb196ed6c9c13003b2ad9becf6c9
Reviewed-on: https://go-review.googlesource.com/c/website/+/334389
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Steve Traut <straut@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

_content/doc/modules/pruning.md

@@ -0,0 +1,3 @@
+<!--{
+	"Redirect": "/doc/modules/managing-dependencies"
+}-->