documentation: add a full table of content for API pages

Hello,

This a improvement request for the OpenColorIO documentation.

# issue

When browsing the API documentation, example https://opencolorio.readthedocs.io/en/latest/api/config.html it is very hard to find the object you are looking for because of the lack of a table of contents providing a summarized overview of the whole page.
We can also notice that all the code objects are missing a permalink anchor to quickly share a direct link to them.


If we have a look at the pages like https://opencolorio.readthedocs.io/en/latest/guides/authoring/authoring.html we can see in the left side-bar that there is some table-of-content going on but it is shallow, i.e. only listing top-level headings and not lower-lever headings like h3.

> ^ screenshot of the doc showing that h2 are not being listed in the sidebar toc

# request

I would really love:

- adding a deep table-of-content (showing all headings level) in an easy to access area in the page (I personally love to that are pinned to the right of your screen and stay in position when you scroll)
- adding permalink for every code object that is being documented

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Let me know if something is not clear.
Best,
Liam.