[yaml/en] Use preferred style; add missing uses

* YAML allows literal tabs in content, but not indentation.
* Two space indent always preferred.
  * Note: YAML dumpers always use 2 space by default.
* '- ...' doesn't need extra indentation.
  * Note: YAML dumpers don't use extra indentation.
* There was no mention of single quoted strings. They are preferred
  and should be used except when double quote semantics are actually
  required. (Best practice).
* Add flow form example for sets: `{a, b, c}`
* Show collapsed form of seq-in-seq: `- - - foo`.

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