Release 1.8.0 preparation: changelog + version bump

[bruAristimunha]

Release 1.8.0 preparation

Prepared on release/1.8.0-prep (branched from origin/develop @ a2180fabb).

Two commits so the changelog can land independently of the exact version
number:

1. doc(whats_new): record #1204 equation formatting fix for 1.8
2. release: bump version to 1.8.0 and date whats_new header (2026-10-06)

Checklist for the release manager

• Confirm the target version number (1.8.0 proposed). If different,
  cherry-pick commit 1 only and bump separately.
• docs/source/whats_new.rst Version 1.8 section audited against every
  PR merged into develop since v1.7.2 (2026-09-14). See
  R1-REPORT.md for the per-PR table. All dataset classes added since
  v1.7.2 are listed. no changelog-labelled PRs
  (fix(download): sanitize '~'-prefixed URL components and migrate existing '~bci' ghosts #1181 DOC: make _corrected_resampled_ttest docstring raw (restores \frac after #1204) #1216 Dataset review W41: record today's 30 NEMAR deposits as pending ids; Thapa2025 session note #1219 Thapa2025: declare sessions per the loader-default rule (b) #1222 test(nemar): drop 15 published deposits from NEMAR_ID_PENDING #1223 FIX Nieto2022: NEMAR id is on003626, not the OpenNeuro id #1224) are intentionally omitted.
• Catalog consistency: pytest moabb/tests/test_metadata.py moabb/tests/test_datasets.py -k 'catalog or nemar_id or metadata'
  → 865 passed, 244 skipped (MOABB_DOWNLOAD_PROVIDER=auto).
  test_metadata.py alone → 286 passed. summary_*.csv row counts
  (imagery 105, p300 68, ssvep 18, cvep 8, imagined_speech 7, rstate 3)
  match the exports in moabb/datasets/__init__.py.
• CI on develop: latest Test ✅ success, Test-braindecode ✅ success,
  Link Check ✅ success, Docs ❌ failure (known OSF 429 flake on
  plot_dreyer_clf_scores_vs_subj_info.py, see report).
• Pre-commit: ran clean locally against both commits.
• Tag v1.8.0 and publish to PyPI (release manager's action; not
  done here).

Not merged

Do not merge this PR until Bruno confirms the version number and tag plan.
No tag has been created.