xbps/repositories/custom.md: document creating a repository

[classabbyamp]

• xbps/repositories/signing.md was merged into custom.md with minor modifications
  • signing.md is now redirected to custom.md
• incorporates @camoz's change to show using 4096-bit RSA keys for signing

closes #641

[Duncaen]

I don't really see the usefulness of documenting xbps-rindex -a or the XBPS_TARGET_ARCH environment variable.
Usually people should just be using xbps-src to create the repositories and then sign them.

[classabbyamp]

Usually people should just be using xbps-src to create the repositories and then sign them

That doesn't help when someone builds locally with xbps-src and uploads the built package to a server to be served. When I was setting up a repository for personal use, I had to search through a bunch of scripts to figure out how to do it without xbps-src's help.

[Duncaen]

I don't really like mixing "custom repository" with the signing stuff, usually people should be using xbps-src and then just sign the resulting repository index.

[classabbyamp]

So should the signing info be put back as a separate page?

[Duncaen]

I think same page is ok, but a separate section for signing and "custom repositories" and maybe instead of calling the xbps-rindex stuff "custom" call it something like "manually maintaining repository".

[camoz]

Can we remove the -des3? Not sure what it does in the command, but DES3 has been broken for some years, and the -des3 in the command was added a long time ago even before DES3 was broken.

[classabbyamp]

I've amended based on feedback.

[camoz]

I should note that I'm not 100% if -des3 can be removed, a confirmation from someone else would be nice :)

Also it should then probably removed from the README.md in void-packages as well, but not via this PR I guess.