Include h3 to docs table of contents

[britneywwc]

Done

• Include h3 headings to table of contents (displayed on the right side of the page). H3 contents should nest their parent H2 headings.

QA

• Go to https://juju-is-564.demos.haus/docs/juju/manage-clouds
• Check that the table of contents on the right side of the page load as expected
• It should catch h2 and h3 headings. H3 headings should nest their corresponding H2 parent headings.

Issue / Card

Fixes WD-12952 and canonical/canonicalwebteam.discourse#187

Screenshots

[if relevant, include a screenshot]

[webteam-app]

Demo

Jenkins

demos.haus

[fasih-mehmood]

LGTM 👍