~dhruvin/builds.sr.ht-guix

Add twelve-tone from GuixRUs as an example
Stop recommending using (guix ci) in channels.scm

Fixes: https://todo.sr.ht/~dhruvin/builds.sr.ht-guix/7
Update README to reflect project changes
Declare cookbook to be available
Finalize the substitute server section
Clarify on non-trivial builds with more than one build environments
Replace `guix install` with `guix package`
Restructure the use section
Restructure the unauthorized substitute servers section
Change the way how channels should be used
Add a note about channel-with-substitutes-available
Add introduction and fix some typos
Flesh out the cookbook
Remove .build.yml from sourcehut build manifests
Write top level sections with essential information
Reorder the Usage section
Clarify the mailing list, bug tracker, and format the README