Get on MELPA; update docs for straight and MELPA

[lastquestion]

• Update docs for Straight
• Update docs for MELPA
• Pass MELPA's lint checks and add them to the makefile

[bqv]

Yes please!

[condy0919]

melpa wanted!

[lastquestion]

I've uploaded a screencast video, so I will add straight.el install docs now, and then run MELPA lint next. I might do #20 at the same time.

[lastquestion]

I have fixed 120+ checkdoc warnings (that was not fun). I need to run package-lint next.

[bqv]

Thanks for the effort!

[baerrach]

Thanks for updating the docs with straight!
use-package with easy cut-and-paste appreciated.