[ENH] add `matches` and `source_entity` to glossary

[Remi-Gau]

• closes #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
• [x] ensure that entities pages are not affected

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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