docs: single toctree

[dgarcia360]

Related issues: #72 #67

This PR rollbacks the custom changes applied to the documentation pages. It also defines the table of contents of the documentation site in a single file (contents.rst).

These modifications will allow us to backport the documentation generation system to other branches with fewer steps: https://github.com/dgarcia360/java-driver/blob/d9505e9fdcd5948611a0b480015cbc3add27b2d2/README-dev.rst#defining-supported-versions

Downsides

Before, all the contents were nicely classified under their parent documentation page. Now, the table of contents will render completely the page.

@lauranovich @tzach If you don't like how the sidebar looks, we could render the different sections as dropdowns with the theme redesign.

Testing this PR

1. Enter the docs directory with cd docs.
2. Run make preview.
3. Review the latest table of contents on the sidebar.

[lauranovich]

Is there a reason Manual is listed as an H1 and an H2?

[dgarcia360]

The branch schema has recently changed, so we don't need to merge this PR.