APKBUILD.5: propose rewriting in scdoc

I am the maintainer of scdoc: https://git.sr.ht/~sircmpwn/scdoc

Maintaining a man page by simply editing the roff macros by hand isn't a very sustainable approach. Additionally, the man pages are currently rendered with a fixed width, which makes them inconvenient to read down the road. I would like the ABUILD.5 man page to become the authoritative definition of APKBUILD syntax (obsoleting the corresponding wiki page), and I would like to expand the abuild docs with additional man pages. It would be easier to achieve these goals moving forward with a more comfortable syntax for writing the man pages in.

scdoc turned two a month ago and has experienced very little churn during its lifetime. It has been in community since Alpine 3.9. It requires only a C99 compiler and a conservative subset of POSIX to build, making it a trivial port to include while bootstrapping an Alpine system. The change would be very conservative when compared to other solutions like asciidoc.