Allow per-page templates.

[carltongibson]

Follow-up to #12/#15 — adding global TOC.

By setting file-wide metadata, before any other markup:

:page_template: sphinx_view/no-globaltoc.html

… it's possibly to switch the template used to render each page, for example to not show the global TOC, on the index page, where it's already defined.

assert doc["meta"]["page_template"] == "sphinx_view/no-globaltoc.html"

1. Store the result of get_doc_json() on self. (As a @property? 🤔)
2. Re-use that in get_template_names() to see if doc["meta"]["page_template"] is set.

• Use that if so.
• Otherwise, self.template_name, as usual.