Display section status based on status-* classes

[WilcoFiers]

This PR adds a function to the script which looks for status indicators on sections, and appends status information after the first heading in that section.

Please note: I've added status indicators into the document to demonstrate the functionality. These do not represent the status as agreed on by the group. I will leave it up to the chairs / editors of WCAG 3 to decide which section should have which status.

A preview of this can be seen here:
https://raw.githack.com/w3c/silver/status-proposal/guidelines/index.html#normative-requirements

[michael-n-cooper]

This is a more comprehensive implementation than I would have done. :) I was expecting status indicators to appear in the for the element, e.g., "Text Alternatives (Exploratory)". This will also cause the status to appear in the TOC. The status para underneath is good to keep as well. I also expected some kind of different styling for the sections, e.g., each status would have some kind of style differentiation in addition to the heading information. But that's a design question outside my area, and something we can improve later.

[WilcoFiers]

Superseded by #577