Allow for a 'backlink' generated solution in Glossary / linking

A notation method to link to a 'in context' location inside the glossary, so that people can navigate from the glossary to the place (or places) where they can read more about a term inside that context. This allows for navigation to and from glossary as well.

We could keep the current :term[...] notation and extend it only for the special cases where we want a glossary backlink.

Examples:

:term[Variable Scope]
:term[test cases]{term="Test Case"}
:term[Variable Scope]{source="#variable-scope-explained"}
:term[test cases]{term="Test Case" source="/docs/tests#test-case-structure"}

Proposal:

• Keep :term[...] as the standard glossary link syntax.
• Keep {term="..."} for alias text.
• Add an optional source="..." attribute for the few cases where a glossary entry should offer a “read more” backlink into the docs.
• If no source is given, no backlink is created in the glossary entry.

Start by locating the implementation of the :term[...] syntax and the glossary generation or rendering path. Trace how the existing term and alias attributes are handled, then verify the proposed source attribute with the examples in this issue; done means glossary entries can optionally link back to the specified context location without changing entries that omit source.