rtb-data packaging and release-please process

[petercorke]

Migrated from tech-debt.md (deleted, see repo history via git log -- tech-debt.md). Groups rtb-data packaging and the release-please/CI process work that's entangled with it.

• Automate rtb-data publishing alongside roboticstoolbox-python releases, only when it actually changed. rtb-data is a separate PyPI package (meshes, xacro/URDF sources) built from rtb-data/, published independently. The main release process (release-please + release.yml) knows nothing about it -- as of 2026-07-03 rtb-data/ on main had drifted well ahead of PyPI (missing rtb-data/pyproject.toml entirely at one point), causing CI failures from xacro files the installed PyPI package didn't have yet. Proposed: a CI step that diffs rtb-data/ against the tree at the last rtb-data publish (tag or recorded SHA); no-op if unchanged, bump + publish if changed. Deliberately not "publish rtb-data on every release" -- it's large (meshes, STL/OBJ) and should stay infrequent.

• Move rtb-data/ into a packages/ folder. Currently sits at repo root alongside the main source tree despite being independently versioned/published. Only two files reference the path (pyproject.toml's sdist.exclude, rtb-data/pyproject.toml itself), so the move is cheap. Worth doing since GRAPHICS-BACKEND.md/SWIFT-MPL-SPLIT.md both describe splitting graphics backends into further sub-packages -- better to establish the packages/ convention with one occupant than reshuffle after two or three exist. Sequence before the publishing-automation item above, so that's built against the final path.

• Credit robot_descriptions in docstrings of the models that load via it. At least 8 model classes (Fetch, Frankie, Jaco, PR2, UR10, UR3, UR5, Valkyrie, YuMi) load via robot_descriptions under the hood, but none of their docstrings say so -- a user reading e.g. UR5's docstring has no way to know the model comes from a third-party package. Add a line naming robot_descriptions with a link to its repo (same URL already used in URDFRobot.py's _RD_URL constant -- reuse it so the two don't drift). Mechanical, low-risk, ~8 files.

• Extend release-please to understand rtb-data as a second component. release-please here only tracks one linear history (. = roboticstoolbox-python) off main's tip -- no concept of releasing from an older point, and no concept of rtb-data/ as a second component. This is why out-of-band maintenance-branch releases (e.g. maintenance/1.3.x off an old tag, bypassing release-please entirely) have been needed when main moved on to in-progress/breaking work before a fix could ship for the currently-released version. Fixing the multi-package gap removes the main reason for doing those. Related operating pattern to keep either way: land CI-health fixes directly against main as small independent PRs, don't stack feature/fix work on top of other unmerged fix PRs -- get main green first. Also note: there is no approval gate on the pypi deployment environment (confirmed via GitHub API) -- a successful build publishes to PyPI immediately once a GitHub Release is created, no dry-run. Always rehearse locally first (python -m build, install into a real throwaway venv).