docs: homebrew formulae fixed, installation docs improved (empty)... [#…

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This commit is to document impactful changes in associated repos:

- The `zsh-abbr` and `zsh-abbr@6` Homebrew formulae successfully install zsh-abbr.
- The GitHub release has an associated working archive.
- The documentation site's manual installation snippet works.
- The documentation site includes solutions for installing with zgen, zit, zr, and zsh4humans.