Update Calcium & Arb homepages

[fingolfin]

Right now https://fredrikj.net/calcium/ and https://fredrikj.net/arb/ give no indication that these libraries have been merged into FLINT, and people still find them unaware of this, and may draw incorrect conclusions from this. E.g. the arb page at the bottom suggests it was last updated 2016...

I would suggest to either turn these into redirects to the flint documentation and/or homepage, or to insert a prominent note on all pages that this is outdated and one should use flint instead.

[albinahlback]

I think a redirect is suitable. For reference, one can just put

<meta http-equiv="refresh" content="0; url=https://flintlib.org/doc/index.html" />

in the header, which should work for all modern browsers.

Arb, Calcium and GR should also be archived on Github. Currently they're not.