x/pkgsite: Syntax Highlight example blocks

[Rhymond]

What is the URL of the page with the issue?

Any page containing example block, e.g. https://pkg.go.dev/io#example-Copy

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.93 Safari/537.36

Screenshot

What did you do?

Open package documentation example

What did you expect to see?

Syntax Highlighted code block.

What did you see instead?

A text blob without syntax highlight

[Rhymond]

#40952 describes similar issue, but with comment code blocks. As I understand correctly it is hard to recognise comment code blocks language, however Example blocks are always (?) written in Go and can be highlighted.