docs: fix dead API link in README and drop link to deleted scanpy_usage repo

[pratikgx]

• Closes: no issue; dead links found while reading the docs.
• Tests not required because: documentation links only.
• Release notes not necessary because: no user-visible behavior change, two dead links repaired.

README: the [api section] reference used api.html; the page is api/index.html (old URL 404s, new one 200).

docs/tutorials/index.md: the "Older tutorials" section pointed only at github.com/scverse/scanpy_usage, which no longer exists (404 with no redirect; the archived scanpy-tutorials repo says tutorials now live in these docs). Removed the section rather than point it somewhere that doesn't hold those tutorials. The seealso block on spatial tutorials that followed it is kept. Happy to re-add the section if the old material lives somewhere I missed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.93%. Comparing base (a41e03f) to head (208dfb2).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4378   +/-   ##
=======================================
  Coverage   81.93%   81.93%           
=======================================
  Files         134      134           
  Lines       13030    13030           
=======================================
  Hits        10676    10676           
  Misses       2354     2354           

Flag	Coverage Δ
hatch-test.low-vers	78.81% <ø> (ø)
hatch-test.pre	81.80% <ø> (ø)

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