Run tidy on code snippets in documentation

[aneeshusa]

For example, #13225 adds new shell code in a sh code fence markdown block. The review comment I made would have been caught by tidy in a regular shell file.

Note that there will probably need to be slight differences in tidy behavior, for example license headers are not required for snippets, shell snippets need not start with set -euo pipefail options, etc.

[highfive]

cc @wafflespeanut

[leikahing]

I can handle this one as well.

[KiChjang]

You seem to have plenty of issues assigned to you already, do you mind finishing them off first before attempting this one?

[leikahing]

@KiChjang - sure thing. I do have an open PR waiting on @aneeshusa which is one of the two issues assigned to me right now.