docs: add connect() and Task.ping() to DJ-2.1 stable surface list

[detail-app]

Section [DJ-2.1] of 13C-Using_Weft_With_Django.md lists the stable
weft.client entry points and Task handle minimum surface. Both connect()
and Task.ping() were shipped but not added to those lists, meaning a
developer implementing the Django integration from this spec would not
know those APIs exist.

A developer following [DJ-2.1] to set up a Django integration would
use WeftClient.from_context() directly and omit connect(), and would
have no spec coverage for programmatic liveness probing via ping().

Introduced by commit 8cd8c7b (@VanL)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.08%. Comparing base (ca3b2e8) to head (fdeffe6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   80.10%   80.08%   -0.02%     
==========================================
  Files         160      160              
  Lines       28829    28829              
==========================================
- Hits        23094    23089       -5     
- Misses       5735     5740       +5     

Flag	Coverage Δ
unittests	80.08% <ø> (-0.02%)	⬇️

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see 6 files with indirect coverage changes

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