fix(docs-infra): convert external links to MatIconButtons (#42584)

This commit changes the anchor elements used for external links to
`MatIconButton`s. While the appearance remains the same (with the
exception of hover/focus styles), this better aligns the styling of
external link icons with other nearby icon buttons (i.e. the theme
toggle) and alows as to simplify the CSS for external links (since much
of their styling is handled by Angular Material).

PR Close #42584

goldens/size-tracking/aio-payloads.json

@@ -3,11 +3,11 @@
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+        "main-es2017": 456646,
         "polyfills-es2017": 55210,
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@@ -15,11 +15,11 @@
     "master": {
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-        "dark-theme": 79419
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   }