Show level 3 headings in "table of content" on the right hand side of the page

[HuangJunye]

Currently only 2 levels of headings are shown. For example: https://qiskit.org/documentation/apidoc/circuit.html

Level 1 is used and only used by the title of the page and therefore only level 2 headings are useful. It should be great to show level 3 headings as well to give more information for users to navigate.

For the same example page: https://qiskit.org/documentation/apidoc/circuit.html, showing level 3 headings will show the subsections under "Quantum Circuit API":

• Quantum Circuit API
  • Quantum Circuit Construction
  • Gates and Instructions
  • Control Flow Operations
  • Parametric Quantum Circuits
  • Random Circuits

[Eric-Arellano]

Fixed by new Furo theme (#425)