docs: point graphics-block docstrings at |GraphicsBlockOptions|

[petercorke]

Summary

• `VehiclePlot`, `ArmPlot`, and `MultiRotorPlot` are all `GraphicsBlock` subclasses (bdsim), so their `**blockargs` now also flows through `GraphicsBlock`'s own kwargs (`movie`, `timestamp`, `timestamp_fmt`, and now `watch` -- see petercorke/bdsim#75) in addition to `Block`'s. Their `blockargs` docstring line pointed only at `Block.init` via `|BlockOptions|`, silently omitting the graphics-level options.
• Added `|GraphicsBlockOptions|` to `conf.py`, mirroring the existing `|BlockOptions|` pattern (same raw-html substitution, redefined per-repo since there's no intersphinx mapping to bdsim's docs today), and pointed these three blocks at it instead.

Companion PR to bdsim#75, which promotes `watch` from a `Scope`-only parameter to a shared `GraphicsBlock.init` parameter -- these three RTB blocks now genuinely accept `watch=True` once that lands, not just document-in-theory.

Test plan

• `pytest tests/ -k "block or bdsim or Vehicle or ArmPlot or MultiRotor"` -- 24 passed, 6 pre-existing skips
• Confirmed `ArmPlot(..., watch=True)` constructs and sets `self.watch = True` correctly against the local bdsim#75 branch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (6e3508b) to head (2c487c7).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #613   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        142     142           
  Lines      13898   13898           
=====================================
  Misses     13898   13898           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.