DOC: Replace reST cross-reference roles in docstrings with plain backticks

[romanlutz]

Why

PyRIT's docs build uses MyST (Markdown-flavoured), not reStructuredText.
reST cross-reference roles like :class:SeedPrompt, :func:oo, :meth:�ar etc. render as raw text under MyST and are inconsistent with the prevailing convention — plain double-backtick code spans inside Google-style docstrings (see e.g. pyrit/datasets/seed_datasets/remote/visual_leak_bench_dataset.py and pyrit/datasets/seed_datasets/remote/harmbench_multimodal_dataset.py).

A reviewer flagged this on pyrit/datasets/seed_datasets/remote/mossbench_dataset.py and it was fixed there in a follow-up commit. This PR audits and fixes the rest of the repo, and adds a style-guide rule to prevent recurrence.

What changed

Audit + fix (Part 1)

Searched pyrit/, tests/, doc/ for reST role syntax in docstrings/comments. Found 55 occurrences across 17 .py files (zero hits in .md/.ipynb):

Role	Count
:class:	25
:func:	11
:meth:	10
:data:	7
:py:func:	2

Every occurrence was rewritten from :role:Name to Name. Pure docstring/comment text changes — no logic or signature changes.

Style rule (Part 2)

Added a "Code references in docstrings" subsection to .github/instructions/style-guide.instructions.md under Documentation Standards → Docstring Format. Its existing applyTo: '**/*.py' front-matter ensures Copilot picks it up for every Python file. The rule:

• Forbids reST cross-reference roles in docstrings/comments.
• Mandates plain double-backtick code spans (matches existing convention).
• Notes the MyST {class}Name alternative only as a rare fallback — the default in PyRIT is plain backticks, not cross-references.

Also added a checklist item to the "Final Checklist" near the end of the file.

Verification

• uv run ruff check — passes
• uv run ruff format --check — all 17 files already formatted
• uv run pre-commit run --files <changed> — all hooks pass (trailing whitespace, EOF, ruff, ty, etc.)
• uv run pytest on changed-module test files — 146/146 passed
• Re-ran the grep — 0 remaining reST role occurrences anywhere under pyrit/, tests/, or doc/.