docs(parsers): keep compile-only API examples

[abraemer]

Summary

• replace redundant doctests across parser and model docs with prose examples, while keeping the PackageParser, RecursionGuard, and register_parser! snippets as no_run examples
• add direct RecursionGuard unit tests so its behavior stays covered by normal tests instead of documentation snippets
• remove the dedicated doctest CI step from .github/workflows/check.yml now that doctests are no longer being used as meaningful test coverage

Issues

• Covers:
• Closes:

Scope and exclusions

• Included: documentation cleanup for former doctests, RecursionGuard test coverage, and CI removal of the doctest step
• Explicit exclusions: the unrelated dirty reference/scancode-toolkit submodule change in the worktree

Intentional differences from Python

• None

Follow-up work

• Created or intentionally deferred: repo docs that still mention doctests were left unchanged because this PR only removes the dedicated CI step and standardizes the remaining examples as compile-only docs

Expected-output fixture changes

• Files changed: none
• Why the new expected output is correct: no golden or expected-output fixtures changed in this branch