Add Arkansas scraper

[riordan]

Closes #788.

Adds warn/scrapers/ar.py and updates docs/sources.md.

How it works

Arkansas publishes WARN notices through an America's Job Link Alliance job center portal, so this reuses the shared warn.platforms.job_center code exactly like Kansas, Maine and Vermont — the scraper is just a thin wrapper supplying the search URL (https://www.arjoblink.arkansas.gov/search/warn_lookups) and a stop_year.

Verification

Ran end-to-end against the live portal: it fetches, paginates and writes ar.csv with the standard job_center columns. The dataset is currently small — the state has posted only a few notices to the portal — but the scraper is correct and will pick up new notices as they're added.

Note

stop_year is set to 2007; happy to adjust if maintainers have better records on how far back Arkansas's portal data goes.

[stucka]

This appears to be a perfectly functional scraper for Arkansas. The problem is Arkansas ... does not actually make the data public. The extant data represents just a couple days of 2019, but has, for example, nothing at all from the deep COVID layoffs and closings around 2020q1 and 2020q2.

I swear somewhere I have an email saying Arkansas doesn't release WARN data as doing so would actually violate state law that protects the privacy of companies, but I can't locate the email at the moment.

This is also highlighting that I've been woefully negligent in updating a list of what places have scrapers, and that needs to get fixed properly.

@riordan , I am so very sorry for the hassle here, and so grateful for your interest. I'm going to try to clean up a few things and will flag this for the documentation side of things.