update OS website links

[svigerske]

With the move from coin-or.org/svn to Github, also the OS website sources have been copied to Github.
What was (and still is) in the /html subdir of the OS svn repo is now in the gh-pages branch of Github - with two exceptions:

• the files in OSRepository/linear/continuous/Kennington/ are now compressed, because Github has a limit of 100MB per file
• empty directories could not be migrated to Github, because git doesn't allow to store empty directories (at least not by default)
  The svn-history of /html could also not be migrated, it seems.

The gh-pages branch is published under https://coin-or.github.io/OS/. This URL essentially replaces https://www.coin-or.org/OS/.

File index.html contained a redirect to the trac page of OS (https://projects.coin-or.org/OS). For the version on Github, I changed this in e5e1ecf to redirect to http://www.optimizationservices.org/. Or should that have pointed to https://github.com/coin-or/OS ?

http://www.optimizationservices.org/ seems to show the content of http://www.coin-or.org/OS/ (starting with welcome.html) in a big frame. It would be good to change this to use https://coin-or.github.io/OS/ instead, or to have a real redirect to https://coin-or.github.io/OS/welcome.html.
Maybe there is also a way to work without redirects and frames and have http://www.optimizationservices.org/ be just a different name for https://coin-or.github.io/OS/, see https://docs.github.com/en/github/working-with-github-pages/configuring-a-custom-domain-for-your-github-pages-site (I have never tried that.).

Also it would be good to check your documentation and website for any other links to the old webpage (www.coin-or.org/OS) or trac (projects.coin-or.org/OS) and replace these by links into the Github world.

[tkralphs]

I changed all the references to GitLab above to Github to avoid confusion.