_content/blog/intro-generics: apply minor touch-ups
_content/blog/intro-generics: apply minor touch-ups

The blog post wasn't listed at go.dev/blog because its metadata
was missing a '---' line at the bottom. Add it. (It was showing
up at go.dev/blog/intro-generics only, with metadata visible as
plain text.)

Fix the URL of the embedded YouTube video.

Point to the moved location of the constraints package in x/exp.

Replace absolute go.dev links with relative ones, so they work
for more users (at golang.google.cn/blog, and so on).

Shorten the links to proposal slightly, using HEAD to refer to the
default branch.

Change-Id: Ie78838a50bb50843e6c242781252d5347063e361
Reviewed-on: https://go-review.googlesource.com/c/website/+/394994
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Eli Bendersky <e83e4a9813852caf164372e4b6bd506b7819d6ab@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website