doc: use stricter indentation checking for docs

First commit:

doc: fix indentation issues in sample code

In preparation for stricter ESLint indentation checking, fix a few
issues in sample code.

Second commit:

doc: use stricter indentation checking for docs

Use stricter ESLint indent checking for sample code in docs.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

doc tools