Move `init` script to top level and update docs
mv lib/git-hub.d/completion share/completion.bash

Move `lib/git-hub.d/completion` to `share/completion.bash`, at Ingy döt
Net’s request, and update references in source files to that file
accordingly.
`lib/git-hub.d/init` → `init` in zsh compl file

In the zsh completion script, `share/zsh-completion/_git-hub`, update a
reference to `lib/git-hub.d/init`, which was moved to repository
top-level, to refer instead to that file’s new location.
Fix misplaced–grave-accent typo in zsh compl file

In the zsh completion script, `share/zsh-completion/_git-hub`, I had
misplaced a U+0060 GRAVE ACCENT, inside a string literal.

This commit fixes this typo.
Fix missing–grave-accent typo in `git-hub.swim`

In the Swim documentation document, `doc/git-hub.swim`, the closing
U+0060 GRAVE ACCENT of an inline code span had been omitted.

This commit fixes this typo.
Quote whole example manually-set (MAN)PATH values

In the Swim documentation document, `doc/git-hub.swim`, an example of
setting one’s `PATH` and `MANPATH` environment variables, so as to
include git-hub’s `lib` and `man` directories, manually (as opposed to
via the `init` script) showed only parts of the example values quoted,
which parts did not include the added elements, i.e., the paths to
git-hub’s `lib` and `man` directories.

These added paths may contain characters requiring quoting, so this
commit changes this example to instead have the whole values be quoted.
Expand `doc/git-hub.swim` § Command Completion

In the Swim documentation document for `git-hub`, `doc/git-hub.swim`,
expand the Command Completion section, in particular, to cover:

 - the generally preferable option of using the `init` script rather
   than enabling completion manually;
 - zsh as well as Bash; and
 - the various file-paths whereat the standard Git completion script for
   Bash might be found.
mv lib/git-hub.d/git-completion share/git-completion.bash

Move `lib/git-hub.d/git-completion` to `share/git-completion.bash`, at
Ingy döt Net’s request, and update references in source files to that
file accordingly.
Account for Homebrew Bash compl file dirs

In lists of file-paths whereat the standard Git completion script for
Bash might be found, include the following two, which may be used by the
package manager Homebrew:

 - `/usr/local/etc/bash_completion.d/git`
 - `~/.homebrew/etc/bash_completion.d/git`