CI: do linkcheck on SPECS docs

[bsipocz]

Some of the specs are full of links to third party pages of tools and other resources. We need to make sure these links don't rot over time.

[jarrodmillman]

We check links with netlify:

specs/netlify.toml

Line 6 in f3fad78

package = "netlify-plugin-checklinks"

[bsipocz]

ah, OK. Also from cron? E.g. I don't expect a link to not work at the time of adding it freshly in the PR.

[stefanv]

No cron checks right now. Link checks are quite expensive, but we could do it once a week or so and file an issue. But I haven't set that up before, so hoping someone else is interested...

[bsipocz]

Oh, sure, I'm not suggesting more frequent checks than weekly/fortnightly.
I do have projects that run linkchecks, but all of those are either rely on jupyter-book or sphinx linkcheck functionality, but I'm sure someone have already found a linkcheck action that works reasonably well so we can just pull the config from an existing project.