Replace Deepnote with Colab in the textbook

[RussTedrake]

Replace Deepnote launches with direct Google Colab links throughout the textbook. All 78 notebooks have conditional package setup, and Meshcat notebooks use the Colab helpers from the published underactuated 2026.9.8 release. Update course and solver instructions, remove obsolete publishing configuration and metadata, and preserve Docker publication as setup/docker/publish.sh.

Update htmlbook to 1851537, which already removes its remaining Deepnote support and fixes IPython notebook exports. Exercise notebooks were edited in the private source repository, pushed to its master, and installed with solutions/install.py; solutions_sha.txt records that revision. Notebook computations are unchanged.

Validation:

• The intro notebook runs all nine original code cells successfully in Google Colab with the published 2026.9.8 package. The separate-window Meshcat link and DisplayMeshcat(meshcat) iframe both render the double pendulum.
• All 96 textbook notebook links and migrated Drake tutorial paths point to existing files.
• Notebook schemas, Python exports, source/install parity, HTML links, HTML metadata, Black, and isort checks pass.
• Python 3.13 local validation passes the public notebook suite and Meshcat/pytest-collection regression tests. Private solution execution passes 22/23; the orbital-transfer SNOPT iteration-limit failure also reproduces in the original private source.
• No Deepnote references remain in the textbook or shared htmlbook checkout.

Package prerequisite #606 is merged and underactuated 2026.9.8 is published on PyPI. This PR is based on master.

Fixes #605.

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