ci: update flint to rust based version

[zeitlinger]

What changed

• consolidated the separate link-check and markdown-lint workflows into a single reusable lint workflow
• switched mise lint tasks to run flint directly and added flint config under .github/config
• removed the old standalone markdownlint config/workflow and updated contributor docs to use the new lint commands
• applied the small markdown formatting fixes required by the new lint flow

Why

Update to rust based flint version.

This aligns the examples repository with the flint-based lint consolidation pattern used in OpenTelemetry Java Instrumentation, reducing duplicate CI wiring and making markdown and link checks run through one entrypoint.

Impact

• contributors now run mise run lint and mise run lint:fix
• CI reports a single lint check instead of separate markdown/link checks
• local link validation is handled through flint configuration

Validation

• mise run lint