add languages to fenced code blocks in docs

[Andriamanitra]

Previously the fenced code blocks in the .md files used language tags inconsistently. This adds languages to all of them (except for ones that just contain plaintext list of actions).

Using jsonc for the json snippets that contain comments allows Github to highlight them properly.

closes #2704

[JoeKar]

Using jsonc for the json snippets that contain comments allows Github to highlight them properly.

Shouldn't we use jsonc directly for all json blocks, regardless of whether we're already using comments or not?
The advantage of this is that any comments added are displayed correctly without requiring further changes to the fenced code block.

[JoeKar]

In runtime/plugins/comment/help/comment.md we still have two json.