encoding/base64: add examples for Encode/Decode
encoding/base64: add examples for Encode/Decode

Fixes #37595

Change-Id: I83e5f6105748a0a9238322a4f7ec4b0bbf61a263
Reviewed-on: https://go-review.googlesource.com/c/go/+/348394
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go