docs: add a five-minute first-contribution path

[dchaudhari7177]

Closes #34.

Adds docs/project/first-contribution.md, linked from the top of CONTRIBUTING.md and from docs/index.md under Project.

Acceptance criteria

• Concise first-contribution page, linked prominently from CONTRIBUTING.md — the link sits directly under the opening line, before Principles.
• Shows clone / editable install / one focused test / full pre-PR checks — in that order.
• Explicitly says examples and fixtures must use synthetic identifiers — stated in full with the reason, not just cross-referenced.
• Does not weaken existing privacy, benchmark or compatibility requirements — see below.
• All documentation links remain valid — 29 relative links across the three touched files checked; none broken.

On not weakening the contract

This was the criterion I was most careful about, since a "quick start" page is exactly where standards get quietly softened. Two deliberate choices:

• The page opens by saying the standards in CONTRIBUTING.md apply to a one-line docs fix exactly as to a new adapter.
• The pre-PR check list is reproduced verbatim and complete — format, lint, mypy, pytest, build, twine — rather than an abbreviated "just run pytest". An abbreviated list would become the de facto standard.

The synthetic-identifier rule is quoted in full with its rationale: fixtures end up in exports, so a real name in a fixture is a privacy incident that outlives the PR.

It also flags the asymmetry a newcomer is most likely to trip on: new behavior requires a failing test before implementation, whereas adding coverage for behavior that already exists does not — which is why the latter is recommended as a starting point.

Verification

Every command in the page was actually run against this tree, not copied from CONTRIBUTING.md on faith:

command	result
pytest tests/test_budget.py -q	12 passed
pytest tests/test_budget.py -q -k reservation	2 passed, 10 deselected
ruff check src tests examples	All checks passed
python examples/shadow_mode.py	exit 0
python examples/universal_runtime.py	exit 0

Note

I deliberately did not link the capability glossary from #37 here, so this PR stands alone and neither depends on merge order. The relevant sentence points at the existing integrations/overview.md#integration-labels instead. Happy to add the glossary link in a follow-up once #37 lands.