feat: consolidate duplicate EXPLAIN templates (before_after_explain.md vs explain_report.md)

[DiogoRibeiro7]

Proposal

Two nearly identical EXPLAIN report templates exist:

• `course/07_week07_indexing/before_after_explain.md`
• `shared/submission_templates/explain_report.md`

Consolidate into one canonical template in `shared/submission_templates/` and have Week 7 README link to it.

Why

Duplicate templates drift over time and confuse contributors about which to update.

Scope / files impacted

• `course/07_week07_indexing/before_after_explain.md` (remove or replace with link)
• `shared/submission_templates/explain_report.md` (keep as canonical)
• `course/07_week07_indexing/README.md`