Several useful links now unavailable on the new website

[eguiraud]

Coming from the forum, e.g.:

• https://root.cern/configuring-xproofd-or-proof-plugin-xrootd/
• https://root.cern/roofit/
• https://root.cern/cms-γ-mva-discrimination/
• https://root.cern/proof/
• https://root.cern/starting-proof/
• https://root.cern/faq/

Maybe we can resurrect some of these, or make them point to corresponding resources in the new website, or have an auto-fallback to https://root.cern/d/ when a page is not found on the new website.

[Axel-Naumann]

We need a server rewrite for URLs that have a replacement in Jekyll. We should not point to /d/ - that's "temporary", we should remove it at some point.

[eguiraud]

Closing as we have lived for 3 years without those links.