Add support for one_page sphinx theme option

[drasmuss]

Motivation and context:

Adds support for the new configuration option added in nengo/nengo-sphinx-theme#59

Interactions with other PRs:

Depends on nengo/nengo-sphinx-theme#59, and downstream repos will be added in the projects that use the one_page option.

How has this been tested?

Added to config notebook, and functionality will be tested in the downstream repos.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.