Fix toctree warnings and render toctree in the HTML sidebar

[aprotyas]

This PR fixes a warning about the library API document not being included in the document, and also fixes the sidebar rendering (table of content shows up in the sidebar correctly).

These two issues are connected because the Sphinx html theme uses the :toctree: directive to create the sidebar.

References:

• Exhale, link to generated documentation API: https://exhale.readthedocs.io/en/latest/usage.html#make-your-documentation-link-to-the-generated-api
• Sphinx rtd theme, table of content options: https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html#table-of-contents-options

Signed-off-by: Abrar Rahman Protyasha abrar@openrobotics.org

[aprotyas]

@wjwwood can you merge the PR?