Add a table of contents to diff_stix's markdown output

[isaisabel]

As a user, I want to be able to access a table of contents for the diff_stix's Markdown output because the output can be very long.

• [ ] The TOC should include anchor links to scroll to the relevant sections.
• [ ] The TOC should adapt to differing contents of the markdown sections, so if the user builds with --contributors it shows up properly
• [ ] The TOC should be opt-in through a flag (like --contributors)