x/website: install from source doesn't exist

[svenwiltink]

https://golang.org/doc/install-source.html (linked from https://golang.org/doc/install) shows the following error:

open REDACTED: file does not exist

[bcmills]

CC @golang/release

[bcmills]

(More evidence for #37047?)

[dmitshur]

This was missed in a recent refactor in CL 296383. We need to add a redirect from the old path at _content/doc/install-source.html:

<!--{
	"Redirect": "/doc/install/source"
}-->

#37047 is going to be more helpful for x/website now that it's going though larger refactors. CC @rsc.

[gopherbot]

Change https://golang.org/cl/324771 mentions this issue: _content/doc: add redirect and fix links to /doc/install/source