Remove API docs headers for functions, exceptions, and methods

[Eric-Arellano]

For functions, methods, and exceptions, we manually create h3 headers:

We do this so that:

1. it shows up in the page ToC, since that is computed based on headers
2. there's the little icon to make an anchor link

But we don't have to have a header for those two benefits; there are other ways we could get that. We can teach our page ToC to understand code objects, and we can add the little anchor icon to the code object directly.

Fixing this will remove duplication and get our design closer to qiskit-sphinx-theme, which we liked:

To implement this, we'll want MDX components like <Function>, which relates to the implementation for #735.

The trickiest part will be the page ToC. We need to make sure we consider the surrounding context like if the parent heading level.

[Eric-Arellano]

We decided to not do this:

1. We're making h3s larger soon and think it actually looks good when combined with the API docs redesign

2. Figuring out the right page ToC is tricky, since it's based on h1-h6
3. Figuring out the search index is tricky, since it differentiates between headers and non-headers
4. It makes semantic sense to have the headers. It's helpful for screen readers and SEO