docs: add command prefix conventions contributor guide

[BioCam]

Summary

Adds a contributor guide documenting standardized verb-prefix conventions for PyLabRobot method names. A method's prefix now signals both its domain (physical machine vs in-memory resource model) and its effect (action / measurement / state-read / state-write), so callers can tell from a name whether a call will round-trip over serial or stay in process.

The guide:

• defines the prefix vocabulary (move_, aspirate_, measure_, read_, request_, set_, get_, update_, assign_, unassign_, ...) and the two tables that pin each prefix to a category,
• lists forbidden synonyms (fetch_, retrieve_, query_, write_, ...) to keep the vocabulary tight,
• gives a decision flow for picking a prefix when writing a new method,
• documents edge cases (cached-vs-fresh reads, methods that act and return, model+machine writes, properties vs methods),
• includes verified real-codebase examples and flags known non-compliant patterns as future harmonization targets.

The guide explicitly tells contributors (and AI assistants) not to silently rename existing methods as part of unrelated work — harmonization is a separate, scoped effort.

Forum discussion that led to this convention: https://discuss.pylabrobot.org/t/standardised-plr-command-prefix-proposal/403

Test plan

• make docs builds without warnings
• New page renders in the contributor guide General section
• All collapsible <details> blocks open/close in the rendered HTML
• Internal links (forum, GitHub trackers) resolve

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