Modify build.py to improve interpretability

[mark-andrews]

Keep names of dictionaries and lists etc in build.py more consistent and identifiable.

Here is a list:

• Instead of a dictionary pages, use pages_dict. This keeps it consistent with content_dict.
• Maybe subpages_info should be subpage_navbar_info because it is a list that is just relevant to the subpages navbar.
• Maybe rename html_rendered as webpage_rendered.
• Maybe html_fragment should be content_rendered or content_fragment.