[ENH] add matches and source_entity to glossary

[Remi-Gau]

• closes [BUG] <matches> is sometimes used in templates but description in the glossary is missing #1748

• https://bids-specification--1781.org.readthedocs.build/en/1781/glossary.html#matches-entities

• https://bids-specification--1781.org.readthedocs.build/en/1781/glossary.html#source_entities-entities

---

• improve filenaming templates with those placeholders to link to glossary.
• link to glossary when they occur in text
• ensure that entities pages are not affected

[Remi-Gau]

adding those in the entities.yml for now

not entirely convinced this is the best place

[oesteban]

perhaps just the glossary? in entities it will interact with the schema, right?

[oesteban]

okay, there's no "glossary.yml" hehehehe sorry for shooting before thinking.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.93%. Comparing base (943c20e) to head (5bf2cc4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1781   +/-   ##
=======================================
  Coverage   87.93%   87.93%           
=======================================
  Files          16       16           
  Lines        1351     1351           
=======================================
  Hits         1188     1188           
  Misses        163      163           

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