Add support for stable doc URLs (#342)

Add support for stable doc URLs (#342)

Doc URLs may change with time because they depend on Doxygen machinery.
This is unfortunate because it is good practice to keep valid URLs
(see: https://www.w3.org/Provider/Style/URI.html).

I could not find a built-in solution in Doxygen, so the solution proposed
here is to maintain a registry of all URLs and manage legacy URLs as
redirections to their canonical page.

This commit adds a registry of URLs that has three functions:
- Check no previous URL is now invalid.
- Add aliases for moved pages.
- Generate redirection pages for aliases. The redirection works with
  a simple <meta http-equiv="refresh"> HTML tag. See:
  https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta#http-equiv

This commit also initialize the URLs registry with current pages and some
redirections needed after recent documentation refactoring.

Finally, the CI is updated to catch any change that invalidate previous
URLs.