all: update <meta> tags for social media

Use the same og:description and description; Signal at least uses description.

Use running gophers for the Twitter image for blog posts.

Adjust the gophers5logo to move the Go where it won't get cut off on Google chat.

Change-Id: I291ad7bf9ea58c0b6af3087213015196434d3163
Reviewed-on: https://go-review.googlesource.com/c/website/+/400434
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

_content/index.md

@@ -1,5 +1,6 @@
 ---
 title: The Go Programming Language
+summary: Go is an open source programming language that makes it easy to build simple, reliable, and efficient software.
 ---
 
 {{$canShare := not googleCN}}

_content/site.tmpl

@@ -11,7 +11,6 @@
   })(window,document,'script','dataLayer','GTM-W8MVQXG');</script>
   <!-- End Google Tag Manager -->
 <meta charset="utf-8">
-<meta name="description" content="Go is an open source programming language that makes it easy to build simple, reliable, and efficient software.">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <meta name="theme-color" content="#00add8">
 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Material+Icons">
@@ -30,9 +29,12 @@
 <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}}
+{{if .summary}}
+<meta name="og:description" content="{{.summary}}">
+<meta name="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:image" content="https://go.dev/doc/gopher/{{if strings.HasPrefix .URL "/blog/"}}runningsquare.jpg{{else}}gopherbelly300.jpg{{end}}">
 <meta name="twitter:card" content="summary">
 <meta name="twitter:site" content="@golang">
 {{if .link -}}