Allow an override for the docs site git URL

[mfurtak]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Description

Folks who use Git via ssh rather than https can include this in their dotfiles to have the fastlane docs updating process work in that way.

export FASTLANE_DOCS_GIT_URL="git@github.com:fastlane/docs.git"

Motivation and Context

Some team members use ssh Git URLs, instead of https. This allows them to override the URL used for the fastlane docs updating process during releases.

[KrauseFx]

👍