Pr-Guidelines Article: remove obsolete information

[linimon]

Summary

As another part of bringing this article into the current century, remove information about obsolete architectures and drivers.

[lwhsu]

I need some more time to check other parts, but in general it looks ok to me.
I would like to check if we want to add riscv64 part, while removing the unsupported arches.
BTW, for mips, it's still supported in 13.x, if we want to do very formally, it may be remove it after 13.x EoL.

[linimon]

Updated per lwhsu comment.

[lwhsu]

documentation/content/en/articles/pr-guidelines/_index.adoc

234

do we want to temporarily keeping this part?

[linimon]

Update per comments.

[imp]

If freebsd-mips still exists, we should retire it.

documentation/content/en/articles/pr-guidelines/_index.adoc

153

Surely remove mips here?

[lwhsu]

In D53646#1225438, @imp wrote:

If freebsd-mips still exists, we should retire it.

The original patch removes it, though my comment is "BTW, for mips, it's still supported in 13.x, if we want to do very formally, it may be remove it after 13.x EoL."
I do not object to strongly remove it now, just mentioned it's ok to be removed later.