chore(credits): drop the "About this work" section — it moves to the About page

The CC BY-SA work-license declaration ("About this work") was on the credits
page, but the About page (built on the parallel feat/accessibility-pass) will
own that content; keeping it in both places would duplicate it. Remove the
section from credits.html and make the footage note self-contained (it still
states the footage is altered — the per-clip "indicate modifications" cue stays
with the attributions). Tests updated to assert the declaration is intentionally
no longer on the credits page. The .declaration CSS rule is left in place for the
About page to reuse.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_e2e_credits.py

@@ -68,7 +68,9 @@ def test_credits_page_lists_video_attributions(browser_page):
     body = page.inner_text("body")
     assert "NASA, ESA, CSA, STScI" in body
     assert "Joe Mabel" in body          # a CC BY-SA clip's required author credit
-    assert "CC BY-SA 4.0" in body       # the Option-D1 derivative declaration
+    # The CC BY-SA work-license declaration ("About this work") moved to the About
+    # page — it is intentionally no longer on the credits page.
+    assert "CC BY-SA 4.0" not in body
 
 
 def test_experience_links_to_credits(browser_page):