feat: opt-in extra sites (Shodan + example checkers) + docs

[dmoney96]

Short summary

Adds opt-in extra-site support and example API-backed checkers.

What changed

• I've been wanting to expand the capabilities of Maigret. It's such a great tool, and adding in other modules, including customizeable yamls, in order to allow for robust, API key compatible platforms like Shodan. If anyone has suggestions for changes or wants me to add anything else, let me know.
• Added maigret_sites_example/ with example checkers (Mastodon API resolver, Shodan checker, etc.).
• Added optional sites_extra.json support and included an example shodan entry.
• Added README_EXTRA.md describing how to opt-in to extras and how to run the integration workflow.
• Unit tests for the new checkers (network calls are mocked).
• CI workflow that runs Shodan integration is present but gated behind workflow_dispatch and a repository secret.

Reviewer checklist

• No API keys/secrets are checked in. (All keys must be configured via GitHub Secrets.)
• Unit tests pass locally (network calls are mocked in unit tests).
• Integration tests are opt-in (manual workflow_dispatch) and require a repo secret (e.g. SHODAN_API_KEY).
• Any dev shims are documented in README_EXTRA.md and removed/prevented from being shipped upstream.
• Extras are disabled by default and enabled at runtime only with MAIGRET_EXTRA_SITES.
• New checkers include unit tests that mock network calls and follow Maigret naming/priority conventions.

Notes — how to run locally

1. Activate the virtualenv:

   source .venv/bin/activate

[JR272700]

Thank you and I'm starting to understand this lol, a lot of research and patience is the trick

[dmoney96]

Thank you and I'm starting to understand this lol, a lot of research and patience is the trick

It is...and in my case, a lot of trial and error. Just trying to expand capabilities a bit and allow for local options like adding local secrets, api keys, etc. so users can customize their deployments in whatever environments they are running in...and in my case, I'm doing it with low vision, so...like I said...work in progress!

[dmoney96]

The PR body has been updated. Please let me know if any further changes are needed.