docs: fix Sphinx docstring formatting and reST syntax errors

[petercorke]

• Fix LaTeX escaping in docstrings (add missing braces, convert to raw strings)
• Correct reST indentation for documentation lists and notes
• Fix code example imports and function references
• Update cross-references using proper Sphinx format (:meth:, ~seealso:)
• Fix parameter name references in docstrings (S→s, math.pi→np.pi)
• Add missing type annotations in parameter documentation
• Remove obsolete documentation files (classes-2d.rst, classes-3d.rst)
• Improve reference formatting for external links

Fixes 19+ files with sphinx build warnings and documentation clarity issues. All changes are non-functional; improve rendering and consistency only.

[petercorke]

Note that classes-2d/3d.rst, shown as deleted, are orphaned documentation stub files. They were likely used for autodoc generation but are no longer referenced in the main documentation structure (not included in any .rst index or toctree).

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[petercorke]

Merge-order suggestion across the currently open PRs, based on a full file-overlap check across all of them. Nothing here is a conflict with anything already merged — these are parallel PRs sharing some source files with each other, so a rebase will likely be needed wherever paths cross, regardless of order.

Recommended order:

1. Fixes to base.plot_box() #179 (fix/base-plot-box) — already approved, CI green, ready now
2. Fix/twist pitch clean #196 (fix/twist-pitch-clean) — isolated to twist.py
3. test: force headless Matplotlib backend for local + CI test runs #197 (fix/headless-tests-locally) — isolated to test files only
4. docs: fix Sphinx docstring formatting and reST syntax errors #195 (docs/sphinx-docstring-fixes) — touches ~18 files, overlaps most of the PRs below; docstring-only/low-risk, so merging it here means the functional PRs below each only rebase against it once (this one)
5. feat(pose,quaternion): add __imatmul__ (@=) operator #199 (feat/imatmul) — only conflicts with refactor: consistent printline() across pose and quaternion classes #200
6. fix(base): export tr2pos2, pos2tr2, tr2adjoint2; rename tradjoint2 #194 (fix/export-base-functions) — conflicts with perf: defer Matplotlib import until something actually plots #198 and refactor: consistent printline() across pose and quaternion classes #200
7. perf: defer Matplotlib import until something actually plots #198 (perf/lazy-matplotlib-import) — conflicts with fix(base): export tr2pos2, pos2tr2, tr2adjoint2; rename tradjoint2 #194 and refactor: consistent printline() across pose and quaternion classes #200
8. refactor: consistent printline() across pose and quaternion classes #200 (refactor/printline) — touches the most shared surface (conflicts with fix(base): export tr2pos2, pos2tr2, tr2adjoint2; rename tradjoint2 #194, perf: defer Matplotlib import until something actually plots #198, feat(pose,quaternion): add __imatmul__ (@=) operator #199); merging it last means it absorbs one final rebase instead of three others rebasing against it