Link technical terms in the three indicator-function definitions #590

The check-informal-definitions check warns no-link-in-definition on three definition subfiles under _subfiles/indicator-functions/: _def-indicator-function.qmd, _def-predicate-indicator.qmd and _def-iverson-bracket.qmd.

The warnings appeared on #587 only because that mechanical PR moved these definitions into their own subfiles unchanged, so the diff-scoped check saw them as added lines. The text itself predates the PR.

Fix: hyperlink each technical term on first use in each definition (for example subset, universal set, proposition, predicate) to its mds definition.