Add missing `assets` dir to published Jazzy docs

[0bmxa]

Content

This aims to fix #3968 which results in missing images on https://realm.github.io/SwiftLint.

Additionally it adds other small improvements to the publishing script, including

• adding comments
• fixing two issues reported by shellcheck (SC2086, SC2164)

Reviewing

For contextual grouping, a commit-by-commit review might be helpful. The main fix is in the last commit.

Testing

I tested the script as such locally, but as this change is mostly a CI fix, we'll only see if it actually works when it ran on CI.

:warning: Be careful when running this manually, as this script touches global git config and ssh keys! :warning: