chore(deps): update themes/hugo-relearn digest to 28fce6b in .gitmodules

This PR contains the following updates:

Package	Update	Change
themes/hugo-relearn	digest	`06e70da` -> `28fce6b`

📅 Schedule: Branch creation - "before 4am" (UTC), Automerge - At any time (no schedule defined).

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coreruleset / documentation