docs(notebooks): modernise two-link-dynamics.ipynb, including the sign convention of gravity

[petercorke]

Notebook renovation, two-link-dynamics.ipynb. It had 3 error cells against main, and a wrong answer that raised no error; it now has none of either. No change to the toolbox.

What changed

• robot.dyntable() is now robot.dynamics().
• ELink, ERobot, ETS.rz() and ETS.tx() are now Link, Robot, ET.Rz() and ET.tx().
• The sign of gravity. The notebook set gravity=[0, g, 0] and said that "accelerating upward at g provides the weight force". With the current convention, gravity is the acceleration due to gravity (default [0, 0, -9.81]) and gravload is the torque needed to hold the arm up, so the arm in this figure (y up) needs [0, -g, 0]. With [0, g, 0] the comparison against the analytic Euler-Lagrange torques printed [-19.62 -4.905] vs [19.62, 4.905]; the outputs that were saved in the notebook before chore(notebooks): generated bootstrap cell, no committed outputs, and a Pyodide install check in CI #848 showed them agreeing. Checked against the Puma 560, whose shoulder gravload at qn is +31.64, as in MATLAB. The prose now says this.
• The standard generated first cell (docs(notebooks): a standard generated first cell, and a shorter bootstrap cell #850) with the modernised note, and the trailing empty cells removed.

After the change, the DH model, the ETS model and the analytic formulas agree for all four configurations, for the gravity and for the velocity terms. Executed headless against main (0 error cells). Not tried in a browser. Written with AI assistance (Claude Code).

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (2d8abf1) to head (41217fd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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##            main    #853   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        146     146           
  Lines      16866   16866           
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  Misses     16866   16866           

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