chore: use consistent case and heading sizes everywhere

[GJZwiers]

This PR improves the consistency of the writing format in the manual chapters, pages and table of contents.

• title case is now used consistenly for chapter headings, the first heading of a manual page, and headings in toc.json.
• subsequent headings of a manual page all use sentence case
• The first heading of a page is now always a single #, and subsequent headings are adjusted accordingly.