Visually differentiate H2 from H3 headings in Metabar Table of Contents

[bmuenzenmeyer]

Visually indicate the difference between an H2 and H3 heading.
This could be indentation or other textual formatting. Whatever it is, make sure it is responsive and does not create a lot of flow problems. For instance, do not indent many many pixels, as that just makes the headings wrap and become longer.

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You should use smaller headings or we should limit depth of what goes on Table of Contents

I like the heading hierarchy of the content - it'd be nice if we visually indicated the indentation level. I think H2 and H3 only.

Look at content like https://nodejs.org/en/learn/test-runner/mocking for similar needs.

Originally posted by @bmuenzenmeyer in #7215 (comment)

[AugustinMauroy]

+1 for that