Ensure correct numerical sorting of TOC when chapters exceed 10

[adiprnm]

This PR solves the order of the TOC when there are more than 10 chapters, so the generated HTML would be like this:

section_0.html section_1.html section_10.html section_2.html section_3.html ...

The write_toc! depends on the order of the section's HTML, thus wrong ordering results in an invalid ToC ordering.

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller
  PRs).
• This PR avoids mixing refactoring changes with feature changes (split into
  two PRs otherwise).
• This PR's title starts is concise and descriptive.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or
  fixes.
• I've updated any docs, .md files, etc… affected by this change.

What

• Make the section number consistent by adding one leading zero if it's below 10
• Start the section number from 1 instead of 0

Why

It's a bug that needs to be solved.

Known limitations

N/A