Docs: Describe the approach to the base template which should include CSS and JS

[thibaudcolas]

Extracted from the list of TODO in the docs’ README.

• The base template should be very minimalistic;
• Not only docs: We need to decide which templates should be rendered in base template and which should be rendered on their own. Currently we render only page templates without using base template, but it's hardcoded. We should, probably, make it configurable.