all: add <meta> tags for social media cards

This should make sharing on Twitter and in chat apps show nice
little cards for go.dev links that are shared. We can revise as needed
once we get some experience seeing them live.

Change-Id: I2fb41afd90193042c881868fe592bd2294208c90
Reviewed-on: https://go-review.googlesource.com/c/website/+/400334
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

_content/site.tmpl

@@ -27,7 +27,14 @@
   })(window,document,'script','dataLayer','GTM-W8MVQXG');</script>
   <!-- End Google Tag Manager -->
 <script src="/js/site.js"></script>
+<meta name="og:url" content="https://go.dev{{.URL}}">
+<meta name="og:title" content="{{.title}}{{if ne .URL "/"}} - The Go Programming Language{{end}}">
 <title>{{.title}}{{if ne .URL "/"}} - The Go Programming Language{{end}}</title>
+{{if .summary}}<meta name="og:description" content="{{.summary}}">{{end}}
+<meta name="og:image" content="https://go.dev/doc/gopher/gopher5logo.jpg">
+<meta name="twitter:image" content="https://go.dev/doc/gopher/gopherbelly300.jpg">
+<meta name="twitter:card" content="summary">
+<meta name="twitter:site" content="@golang">
 {{if .link -}}
 <meta http-equiv="refresh" content="0; url={{.link}}">
 {{end -}}