Broken SIG links

Our [SIG YAML files](https://github.com/acl-org/acl-anthology/tree/master/data/yaml/sigs) contain many external links to proceedings outside the Anthology (`rg http` gives 77 such links at the moment). I have spot-checked some and found links that, according to the Wayback Machine, have been defunct for _over twenty years_.

What should be our policy here? Remove those links? Replace with an archive.org link if available?

Related: #812

Start by reviewing data/yaml/sigs and the related #812 discussion, then run rg http there to inventory the external proceedings links and check representative URLs against the Wayback Machine. Before editing, agree whether dead links should be removed or replaced with archive.org links; the issue is done when that policy is decided and applied consistently.

由索引模型根据 Issue 内容生成。