move cgi to nimble packages

[ringabout]

https://github.com/nim-lang/cgi

I will start with this simple package, and apply the process to other packages.

[ringabout]

I decided to remove cgi instead of giving an error because the former solution doesn't require users to change the code after installing cgi. The latter solution enforces using pkg/cgi. We change the name though.

[ringabout]

For karax karaxnim/karax#233

[ringabout]

for rosencrantz andreaferretti/rosencrantz#30

[metagn]

Also remove from doc/lib.md (shouldn't cause conflicts)