Avoid duplicate anchors when generating table of contents in the Handbook

[garrett-eclipse]

Hello,

As suggested by @sergey in Slack I'm opening this ticket to address the duplicate anchors that can occur when the same heading is used multiple times in a document.

For instance originally the Working with Patches document had several 'Applying a Patch' references which all anchored to '#applying-a-patch' so they'd all lead to the first item.

I've addressed this by manually making all the headings unique for this page;
​https://make.wordpress.org/core/handbook/tutorials/working-with-patches/

It was indicated the code for this is located here;
https://meta.trac.wordpress.org/browser/sites/trunk/wordpress.org/public_html/wp-content/plugins/handbook/inc/table-of-contents.php

All the best,
Cheers

[coffee2code]

In 8899:

Handbook plugin: Prevent duplicate anchors when generating table of contents links from heading tags.

Previously, if text for a heading was used more than once in the page, the same anchor would be created for each heading, making the second (and subsequent) duplicates untargetable and the toc links would all point to the first instance.

Fixes #4406.