Support Backlinks from definition to usage

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User Story

As a user I want to have a link from the definition of a term to its occurrences in text.

Alternatively the software may offer an option to generate an index document which lists all glossary terms (without their detailed definition). Every term links to the term's glossary definition as well as to the parent section of the paragraph where it was found in text. The link to its definition may always be the first one. Multiple occurrences in the same section of a document only generate one section link in the index.

For example an index could look like

Index

• Hercules mentionend in
  • The Ancient Greeks
  • Greek Mythodology

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This could require a breaking change since the section headings of non-glossary documents must be made navigable. To be navigable they must be wrapped into links as we do for glossary headings already, e.g. with remark-autolink-headings. However, this could potentially alter manually created, navigable headings and their URL fragments. Changing the fragment could result in broken cross-links or bookmarks (they'll still find the page but make the HTML viewer no longer scroll to the correct position).