loader: Create loader_simp(8) to document simple version of loader

loader_simp is a much simplified version of loader that will process a
linear sequence of commands from loader.rc. It has neither Forth nor Lua
built in and is much smaller. Document it. This is largely copied from
loader.8 since it implements those built-in commands. Future revisions
will fix this duplication.

Sponsored by: Netflix

(cherry picked from commit 9873c807dac1f8b83b9f7bc9a8d4e318ea295b55)