Fix MkDocs build from template instances

[matt-graham]

Resolves #515

• Updates mkdocs.yml configuration in template to use structure required by current MkDocs.
• Fixes TOML tox configuration for docs environment to use required array of arrays syntax.

Leaving this as a draft for now as we probably want to add a test for building MkDocs site from a template instance to catch things like this in future, ideally by running tox -e docs as well as mkdocs build to also catch error in tox configuration. I am not sure whether we would want to add tox as a development dependency here, or create a temporary virtual environment as suggested also in #516, install tox there and test running tox -e docs (plus potentially default test environment)?

[matt-graham]

If we do go with including pytest-venv as a test dependency in #519 then that would be an easy way to also test running mkdocs build by installing mkdocs and tox in a temporary venv.

[paddyroddy]

Looks good to me