Deprecate old website and redirect to WAI website

[Jym77]

It seems that the old https://act-rules.github.io/ website is no more updated, which is fine since it has mostly been replaced by the new one (https://www.w3.org/WAI/standards-guidelines/act/rules/).

This, however, still creates a bit of confusion with people who keep using the old website (and sometimes notice discrepancies with the implementations). There are a few pages (guidelines, design docs, …) that are not in the WAI website.

We should probably completely remove the old website.

• Pages that also exists in the WAI website should be replaced by a "301 Permanent redirect" to the WAI pages.
• The few other pages should maybe be moved to the W3C CG pages; and then also replaced by a 301.
• The old site can then be frozen away…

[Jym77]

Should be done in https://github.com/act-rules/act-rules-web